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/gc/gsubr.go b/src/cmd/compile/internal/gc/gsubr.go
index 14dc927..2c575f3 100644
--- a/src/cmd/compile/internal/gc/gsubr.go
+++ b/src/cmd/compile/internal/gc/gsubr.go
@@ -602,6 +602,13 @@
 var reg [100]int       // count of references to reg
 var regstk [100][]byte // allocation sites, when -v is given
 
+func GetReg(r int) int {
+	return reg[r-Thearch.REGMIN]
+}
+func SetReg(r, v int) {
+	reg[r-Thearch.REGMIN] = v
+}
+
 func ginit() {
 	for r := range reg {
 		reg[r] = 1