Also fix a mistake in previous CL about x8 and x16 shifts:
the shift needs ZeroExt.

Progress on SSA for ARM. Still not complete.

Updates #15365.

Change-Id: Ibc352760023d38bc6b9c5251e929fe26e016637a
Reviewed-on: https://go-review.googlesource.com/23486Reviewed-by: David Chase <drchase@google.com>