cmd/compile: avoid zero extensions after 32-bit shifts
zeroUpper32Bits wasn't checking for shift-extension ops. This would not
check shifts that were marking as bounded by prove (normally, shifts
are wrapped in a sequence that ends with an ANDL, and zeroUpper32Bits
would see the ANDL).
This produces no changes on generated output right now, but will be
important once CL196679 lands because many shifts will be marked
as bounded, and lower will stop generating the masking code sequence
around them.
Change-Id: Iaea94acc5b60bb9a5021c9fb7e4a1e2e5244435e
Reviewed-on: https://go-review.googlesource.com/c/go/+/226338 Reviewed-by: Keith Randall <khr@golang.org>