cmd/compile: cleanup OREAL/OIMAG constant folding

Based on suggestion from gri@ on golang.org/cl/166980.

Passes toolstash-check.

Change-Id: I79b66bb09b5635f3a9daecaa5d605b661a0ab108
Reviewed-on: https://go-review.googlesource.com/c/go/+/167501
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -676,17 +676,17 @@ func evconst(n *Node) {
 	case OREAL, OIMAG:
 		if nl.Op == OLITERAL {
 			var re, im *Mpflt
-			switch consttype(nl) {
-			case CTINT, CTRUNE:
+			switch u := nl.Val().U.(type) {
+			case *Mpint:
 				re = newMpflt()
-				re.SetInt(nl.Val().U.(*Mpint))
+				re.SetInt(u)
 				// im = 0
-			case CTFLT:
-				re = nl.Val().U.(*Mpflt)
+			case *Mpflt:
+				re = u
 				// im = 0
-			case CTCPLX:
-				re = &nl.Val().U.(*Mpcplx).Real
-				im = &nl.Val().U.(*Mpcplx).Imag
+			case *Mpcplx:
+				re = &u.Real
+				im = &u.Imag
 			default:
 				Fatalf("impossible")
 			}