cmd/5g, etc: prepare to unexport gc.Mp*

Remove all uses of Mp* outside of the gc package.

A subsequent, automated commit in the Go 1.6
cycle will unexport all Mp* functions and types.

No functional changes. Passes toolstash -cmp.

Change-Id: Ie1604cb5b84ffb30b47f4777d4235570f2c62709
Reviewed-on: https://go-review.googlesource.com/9263
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/src/cmd/8g/ggen.go b/src/cmd/8g/ggen.go
index 5902552..baa1b64 100644
--- a/src/cmd/8g/ggen.go
+++ b/src/cmd/8g/ggen.go
@@ -216,9 +216,9 @@
 	check := 0
 	if gc.Issigned[t.Etype] {
 		check = 1
-		if gc.Isconst(nl, gc.CTINT) && gc.Mpgetfix(nl.Val.U.Xval) != -1<<uint64(t.Width*8-1) {
+		if gc.Isconst(nl, gc.CTINT) && nl.Int() != -1<<uint64(t.Width*8-1) {
 			check = 0
-		} else if gc.Isconst(nr, gc.CTINT) && gc.Mpgetfix(nr.Val.U.Xval) != -1 {
+		} else if gc.Isconst(nr, gc.CTINT) && nr.Int() != -1 {
 			check = 0
 		}
 	}
@@ -391,7 +391,7 @@
 		var n1 gc.Node
 		gc.Regalloc(&n1, nl.Type, res)
 		gmove(&n2, &n1)
-		sc := uint64(gc.Mpgetfix(nr.Val.U.Xval))
+		sc := uint64(nr.Int())
 		if sc >= uint64(nl.Type.Width*8) {
 			// large shift gets 2 shifts by width-1
 			gins(a, ncon(uint32(w)-1), &n1)