cmd/compile: restore constant folding optimization within OADDSTR

Change-Id: Ib55f2458c75aee49302f0dd4e2a819f9931a5ed3
Reviewed-on: https://go-review.googlesource.com/c/go/+/166982
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -643,17 +643,6 @@ func evconst(n *Node) {
 					i2++
 				}
 
-				// Hack to appease toolstash. Because
-				// we were checking isGoConst early
-				// on, we wouldn't collapse adjacent
-				// string constants unless the entire
-				// string was a constant.
-				//
-				// TODO(mdempsky): Remove in next commit.
-				if i1 != 0 || i2 != len(s) {
-					return
-				}
-
 				nl := *s[i1]
 				nl.Orig = &nl
 				nl.SetVal(Val{strings.Join(strs, "")})