cmd/5g: improve shift code generation

This CL is a backport of 6012049 which improves code
generation for shift operations.

benchmark       old ns/op    new ns/op    delta
BenchmarkLSL            9            5  -49.67%
BenchmarkLSR            9            4  -50.00%

R=golang-dev, minux.ma, r, rsc
CC=golang-dev
https://golang.org/cl/6813045

src/cmd/5g/ggen.c

@@ -407,7 +407,9 @@ cgen_asop(Node *n)
 hard:
 	n2.op = 0;
 	n1.op = 0;
-	if(nr->ullman >= nl->ullman || nl->addable) {
+	if(nr->op == OLITERAL) {
+		// don't allocate a register for literals.
+	} else if(nr->ullman >= nl->ullman || nl->addable) {
 		regalloc(&n2, nr->type, N);
 		cgen(nr, &n2);
 		nr = &n2;