cmd/internal/gc: inline x := y.(*T) and x, ok := y.(*T)

These can be implemented with just a compare and a move instruction.
Do so, avoiding the overhead of a call into the runtime.

These assertions are a significant cost in Go code that uses interface{}
as a safe alternative to C's void* (or unsafe.Pointer), such as the
current version of the Go compiler.

*T here includes pointer to T but also any Go type represented as
a single pointer (chan, func, map). It does not include [1]*T or struct{*int}.
That requires more work in other parts of the compiler; there is a TODO.

Change-Id: I7ff681c20d2c3eb6ad11dd7b3a37b1f3dda23965
Reviewed-on: https://go-review.googlesource.com/7862
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/src/cmd/internal/gc/go.go b/src/cmd/internal/gc/go.go
index c33664f..6dd17c1 100644
--- a/src/cmd/internal/gc/go.go
+++ b/src/cmd/internal/gc/go.go
@@ -490,6 +490,7 @@
 var debugstr string
 
 var Debug_checknil int
+var Debug_typeassert int
 
 var importmyname *Sym // my name for package