cmd/compile: eliminate dead code

During walkexpr, we were assessing whether shifts were bounded.
However, that information was dropped on the floor during SSA conversion.
The SSA backend already finds all bounded shifts that walkexpr could have,
and at negligible extra cost (0.02% in alloc, CPU undetectable).

Change-Id: Ieda1af1a2a3ec99bfdc2b0b704c9b80ce8a34486
Reviewed-on: https://go-review.googlesource.com/c/148897


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>