cmd/compile: add test for fixed ICE on untyped conversion

The ICE reported as #33308 was fixed by a related CL; this change adds
a regression test with the crasher.

Fixes #33308

Change-Id: I3260075dbe3823b56b8825e6269e57a0fad185a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/243458
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue33308.go

@@ -0,0 +1,12 @@
+// errorcheck
+
+// Copyright 2020 The Go Authors. All rights reserved. Use of this
+// source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+// Test that the compiler does not crash on a []byte conversion of an
+// untyped expression.
+package p
+
+var v uint
+var x = []byte((1 << v) + 1) // ERROR "cannot convert"