runtime: always pass type to mallocgc when allocating scannable memory
We allocate scannable memory w/o type only in few places in runtime.
All these cases are not-performance critical (e.g. G or finq args buffer),
and in long term they all need to go away.
It's not worth it to have special code for this case in mallocgc.
So use special fake "notype" type for such allocations.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://golang.org/cl/127450044

src/pkg/runtime/malloc.go

diff --git a/src/pkg/runtime/malloc.go b/src/pkg/runtime/malloc.go
index 84c69ab..152b3b6 100644
--- a/src/pkg/runtime/malloc.go
+++ b/src/pkg/runtime/malloc.go
@@ -225,66 +225,56 @@
 			*xbits |= (bitsPointer << 2) << shift
 			goto marked
 		}
-		if typ != nil && (uintptr(typ.gc[0])|uintptr(typ.gc[1])) != 0 && uintptr(typ.size) > ptrSize {
-			if typ.kind&kindGCProg != 0 {
-				nptr := (uintptr(typ.size) + ptrSize - 1) / ptrSize
-				masksize := nptr
-				if masksize%2 != 0 {
-					masksize *= 2 // repeated
-				}
-				masksize = masksize * pointersPerByte / 8 // 4 bits per word
-				masksize++                                // unroll flag in the beginning
-				if masksize > maxGCMask && typ.gc[1] != 0 {
-					// If the mask is too large, unroll the program directly
-					// into the GC bitmap. It's 7 times slower than copying
-					// from the pre-unrolled mask, but saves 1/16 of type size
-					// memory for the mask.
-					mp := acquirem()
-					mp.ptrarg[0] = x
-					mp.ptrarg[1] = unsafe.Pointer(typ)
-					mp.scalararg[0] = uint(size)
-					mp.scalararg[1] = uint(size0)
-					onM(&unrollgcproginplace_m)
-					releasem(mp)
-					goto marked
-				}
-				ptrmask = (*uint8)(unsafe.Pointer(uintptr(typ.gc[0])))
-				// Check whether the program is already unrolled.
-				if uintptr(goatomicloadp(unsafe.Pointer(ptrmask)))&0xff == 0 {
-					mp := acquirem()
-					mp.ptrarg[0] = unsafe.Pointer(typ)
-					onM(&unrollgcprog_m)
-					releasem(mp)
-				}
-				ptrmask = (*uint8)(add(unsafe.Pointer(ptrmask), 1)) // skip the unroll flag byte
-			} else {
-				ptrmask = (*uint8)(unsafe.Pointer(&typ.gc[0])) // embed mask
+		if typ.kind&kindGCProg != 0 {
+			nptr := (uintptr(typ.size) + ptrSize - 1) / ptrSize
+			masksize := nptr
+			if masksize%2 != 0 {
+				masksize *= 2 // repeated
 			}
-			if size == 2*ptrSize {
-				xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-				*xbitsb = *ptrmask | bitBoundary
+			masksize = masksize * pointersPerByte / 8 // 4 bits per word
+			masksize++                                // unroll flag in the beginning
+			if masksize > maxGCMask && typ.gc[1] != 0 {
+				// If the mask is too large, unroll the program directly
+				// into the GC bitmap. It's 7 times slower than copying
+				// from the pre-unrolled mask, but saves 1/16 of type size
+				// memory for the mask.
+				mp := acquirem()
+				mp.ptrarg[0] = x
+				mp.ptrarg[1] = unsafe.Pointer(typ)
+				mp.scalararg[0] = uint(size)
+				mp.scalararg[1] = uint(size0)
+				onM(&unrollgcproginplace_m)
+				releasem(mp)
 				goto marked
 			}
-			te = uintptr(typ.size) / ptrSize
-			// If the type occupies odd number of words, its mask is repeated.
-			if te%2 == 0 {
-				te /= 2
+			ptrmask = (*uint8)(unsafe.Pointer(uintptr(typ.gc[0])))
+			// Check whether the program is already unrolled.
+			if uintptr(goatomicloadp(unsafe.Pointer(ptrmask)))&0xff == 0 {
+				mp := acquirem()
+				mp.ptrarg[0] = unsafe.Pointer(typ)
+				onM(&unrollgcprog_m)
+				releasem(mp)
 			}
+			ptrmask = (*uint8)(add(unsafe.Pointer(ptrmask), 1)) // skip the unroll flag byte
+		} else {
+			ptrmask = (*uint8)(unsafe.Pointer(&typ.gc[0])) // embed mask
 		}
 		if size == 2*ptrSize {
 			xbitsb := (*uint8)(add(unsafe.Pointer(xbits), shift/8))
-			*xbitsb = (bitsPointer << 2) | (bitsPointer << 6) | bitBoundary
+			*xbitsb = *ptrmask | bitBoundary
 			goto marked
 		}
+		te = uintptr(typ.size) / ptrSize
+		// If the type occupies odd number of words, its mask is repeated.
+		if te%2 == 0 {
+			te /= 2
+		}
 		// Copy pointer bitmask into the bitmap.
 		for i := uintptr(0); i < size0; i += 2 * ptrSize {
-			v := uint8((bitsPointer << 2) | (bitsPointer << 6))
-			if ptrmask != nil {
-				v = *(*uint8)(add(unsafe.Pointer(ptrmask), ti))
-				ti++
-				if ti == te {
-					ti = 0
-				}
+			v := *(*uint8)(add(unsafe.Pointer(ptrmask), ti))
+			ti++
+			if ti == te {
+				ti = 0
 			}
 			if i == 0 {
 				v |= bitBoundary