[dev.regabi] cmd/compile: refactor type/value assertions

Small refactoring to make subsequent CLs clearer.

Passes toolstash-check.

Change-Id: I1a6ae599f491220d44aaabae0b7bed4aff46ee92
Reviewed-on: https://go-review.googlesource.com/c/go/+/272651
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

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

@@ -251,6 +251,9 @@ func (n *Node) SetVal(v Val) {
 		Dump("have Opt", n)
 		Fatalf("have Opt")
 	}
+	if n.Op == OLITERAL {
+		assertRepresents(n.Type, v)
+	}
 	n.SetHasVal(true)
 	n.E = v.U
 }