cmd/compile: apply constant folding to ORUNESTR

ORUNESTR represents the special case of integer->string conversion. If
the integer is a constant, then the string is a constant too, so
evconst needs to perform constant folding here.

Passes toolstash-check.

Fixes #34563.

Change-Id: Ieab3d76794d8ce570106b6b707a4bcd725d156e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/197677
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -591,7 +591,7 @@ func evconst(n *Node) {
 			setconst(n, shiftOp(nl.Val(), op, nr.Val()))
 		}
 
-	case OCONV:
+	case OCONV, ORUNESTR:
 		if okforconst[n.Type.Etype] && nl.Op == OLITERAL {
 			setconst(n, convertVal(nl.Val(), n.Type, true))
 		}

test/const.go

@@ -24,6 +24,10 @@ const (
 
 	ctrue  = true
 	cfalse = !ctrue
+
+	// Issue #34563
+	_ = string(int(123))
+	_ = string(rune(456))
 )
 
 const (