[dev.regabi] cmd/compile: move okforconst into its own declaration

It needs to move into package ir, and we do not want all the rest.

Change-Id: Ibcfa1ebc0e63fe3659267bf2fa7069e8a93de4e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/272930
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/go.go

@@ -145,9 +145,10 @@ var (
 	okforcap   [NTYPE]bool
 	okforlen   [NTYPE]bool
 	okforarith [NTYPE]bool
-	okforconst [NTYPE]bool
 )
 
+var okforconst [NTYPE]bool
+
 var (
 	okfor [OEND][]bool
 	iscmp [OEND]bool