When we're doing a compare of load-AND-constant to 0

When we're doing a compare of load-AND-constant to 0
(e.g. a bitfield test) narrow the load as much as possible.
The has the potential to avoid unnecessary partial-word
load-after-store conflicts, which cause stalls on several targets.
Also a size win on x86 (testb vs testl).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58825 91177308-0d34-0410-b5e6-96231b3b80d8