[X86] Don't select (cmp (and, imm), 0) to testw

Summary:
X86ISelDAGToDAG tries to analyze ANDs compared with 0 to optimize to narrower immediates using subregisters.

I don't think we should be optimizing to 16-bit test instructions. It goes against our normal behavior of promoting i16 operations to i32. It only saves one byte due to the need to add a 0x66 prefix. I think it would also be subject to a length changing prefix penalty in the decoders on Intel CPUs.

Reviewers: RKSimon, zvi, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38273

llvm-svn: 314474