Bit-test rules failed to match when matching the highest bit
of a word because operands in SSA are signed int64. Fix
them by treating them as unsigned (and correctly handling
32-bit operands as well).

Tests will be added in next CL.

Change-Id: I491c4e88e7e2f87e9bb72bd0d9fa5d4025b90736
Reviewed-on: https://go-review.googlesource.com/94765Reviewed-by: Keith Randall <khr@golang.org>