cmd/compile: checkwidthj all expressions in walk

Instead of playing whack-a-mole finding all
the non-dowidth'd expressions that can sneak
out of the frontend and then deciding on
just the right place to handle them,
use a big hammer.

Fixes #20152

Change-Id: Id452d9e8c4e9585216bd8bf0e0004c85aba4f9f7
Reviewed-on: https://go-review.googlesource.com/42021
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -461,6 +461,15 @@ func walkexpr(n *Node, init *Nodes) *Node {
 		return n
 	}
 
+	// Eagerly checkwidth all expressions for the back end.
+	if n.Type != nil && !n.Type.WidthCalculated() {
+		switch n.Type.Etype {
+		case TBLANK, TNIL, TIDEAL:
+		default:
+			checkwidth(n.Type)
+		}
+	}
+
 	if init == &n.Ninit {
 		// not okay to use n->ninit when walking n,
 		// because we might replace n with some other node