[dev.ssa] cmd/compile/ssa: fix string compilation

Compilation of f_ssa was broken by CL 10929.

This CL does not include tests because
I have a work in progress CL that will catch
this and much more.

package p

func f_ssa() string {
	return "ABC"
}

Change-Id: I0ce0e905e4d30ec206cce808da406b9b7f0f38e9
Reviewed-on: https://go-review.googlesource.com/11136Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -478,7 +478,7 @@ func (s *state) assign(op uint8, left *Node, right *Node) {
 		t := left.Type
 		switch {
 		case t.IsString():
-			val = s.entryNewValue0(ssa.OpConst, left.Type)
+			val = s.entryNewValue0A(ssa.OpConst, left.Type, "")
 		case t.IsInteger():
 			val = s.entryNewValue0(ssa.OpConst, left.Type)
 		case t.IsBoolean():