cmd/compile: s/flag_race/instrumenting/ in OCONVNOP check

This check seems to apply to all code instrumentation, not just -race.

Fixes #14589.

Change-Id: I16ae07749ede7c1e6ed06f472711638d195034ac
Reviewed-on: https://go-review.googlesource.com/20113
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>

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

@@ -1481,7 +1481,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 			return nil
 		}
 
-		if flag_race != 0 {
+		if instrumenting {
 			// These appear to be fine, but they fail the
 			// integer constraint below, so okay them here.
 			// Sample non-integer conversion: map[string]string -> *uint8