cmd/compile: handle OCONV[NOP] in samesafeexpr

This increases the effectiveness of the
"integer-in-range" CL that follows.

Change-Id: I23b7b6809f0b2c25ed1d59dd2d5429c30f1db89c
Reviewed-on: https://go-review.googlesource.com/27651
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3186,9 +3186,14 @@ func samesafeexpr(l *Node, r *Node) bool {
 	case ODOT, ODOTPTR:
 		return l.Sym != nil && r.Sym != nil && l.Sym == r.Sym && samesafeexpr(l.Left, r.Left)
 
-	case OIND:
+	case OIND, OCONVNOP:
 		return samesafeexpr(l.Left, r.Left)
 
+	case OCONV:
+		// Some conversions can't be reused, such as []byte(str).
+		// Allow only numeric-ish types. This is a bit conservative.
+		return issimple[l.Type.Etype] && samesafeexpr(l.Left, r.Left)
+
 	case OINDEX:
 		return samesafeexpr(l.Left, r.Left) && samesafeexpr(l.Right, r.Right)