cmd/compile: remove stale register use array

The reg[] array in .../gc is where truth lies.  The copy in .../ARCH
is incorrect as it is mostly not updated to reflect regalloc decisions.

This bug was introduced in the rewrite
https://go-review.googlesource.com/#/c/7853/.  The new reg[] array was
introduced in .../gc but not all of the uses were removed in the
.../ARCH directories.

Fixes #12133

Change-Id: I6364fc403cdab92d802d17f2913ba1607734037c
Reviewed-on: https://go-review.googlesource.com/13630
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/src/cmd/compile/internal/x86/ggen.go b/src/cmd/compile/internal/x86/ggen.go
index dabc139..ae9881d 100644
--- a/src/cmd/compile/internal/x86/ggen.go
+++ b/src/cmd/compile/internal/x86/ggen.go
@@ -319,7 +319,7 @@
 }
 
 func savex(dr int, x *gc.Node, oldx *gc.Node, res *gc.Node, t *gc.Type) {
-	r := int(reg[dr])
+	r := gc.GetReg(dr)
 	gc.Nodreg(x, gc.Types[gc.TINT32], dr)
 
 	// save current ax and dx if they are live
@@ -408,7 +408,7 @@
 	var oldcx gc.Node
 	var cx gc.Node
 	gc.Nodreg(&cx, gc.Types[gc.TUINT32], x86.REG_CX)
-	if reg[x86.REG_CX] > 1 && !gc.Samereg(&cx, res) {
+	if gc.GetReg(x86.REG_CX) > 1 && !gc.Samereg(&cx, res) {
 		gc.Tempname(&oldcx, gc.Types[gc.TUINT32])
 		gmove(&cx, &oldcx)
 	}