cmd/compile: cull dead code

The special case for ODOTPTR to handle zero-width fields is unneeded.
It is an artifact of the old backend, from which time this code dates.
The Node to SSA converter is careful to insert a nil check.
This is tested in test/nilptr2.go, among other places.

Passes toolstash-check.

Change-Id: I6c1d99f7ff5abdae9aa08ee047dc088a3fe8dc3c
Reviewed-on: https://go-review.googlesource.com/c/148828
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -493,7 +493,7 @@ opswitch:
 		n.Left = walkexpr(n.Left, init)
 		n.Right = walkexpr(n.Right, init)
 
-	case ODOT:
+	case ODOT, ODOTPTR:
 		usefield(n)
 		n.Left = walkexpr(n.Left, init)
 
@@ -508,17 +508,6 @@ opswitch:
 			n.List.Set1(itabname(n.Type, n.Left.Type))
 		}
 
-	case ODOTPTR:
-		usefield(n)
-		if n.Op == ODOTPTR && n.Left.Type.Elem().Width == 0 {
-			// No actual copy will be generated, so emit an explicit nil check.
-			n.Left = cheapexpr(n.Left, init)
-
-			checknil(n.Left, init)
-		}
-
-		n.Left = walkexpr(n.Left, init)
-
 	case OLEN, OCAP:
 		if isRuneCount(n) {
 			// Replace len([]rune(string)) with runtime.countrunes(string).