cmd/compile/internal/ssa: refactor ARM64 address folding

These patterns are the only uses of isArg and isAuto, and they all
follow a common pattern too. Extract out so that we can more easily
tweak the interface for isArg/isAuto.

Passes toolstash -cmp for linux/arm64.

Change-Id: I9c509dabdc123c93cb1ad2f34fe8c12a9f313f6d
Reviewed-on: https://go-review.googlesource.com/40490
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/rewrite.go

@@ -284,6 +284,20 @@ func isAuto(s interface{}) bool {
 	return ok
 }
 
+func fitsARM64Offset(off, align int64, sym interface{}) bool {
+	// only small offset (between -256 and 256) or offset that is a multiple of data size
+	// can be encoded in the instructions
+	// since this rewriting takes place before stack allocation, the offset to SP is unknown,
+	// so don't do it for args and locals with unaligned offset
+	if !is32Bit(off) {
+		return false
+	}
+	if align == 1 {
+		return true
+	}
+	return !isArg(sym) && (off%align == 0 || off < 256 && off > -256 && !isAuto(sym))
+}
+
 // isSameSym returns whether sym is the same as the given named symbol
 func isSameSym(sym interface{}, name string) bool {
 	s, ok := sym.(fmt.Stringer)