[X86] Add BLSI to isUseDefConvertible.

Summary:
BLSI sets the C flag is the input is not zero. So if its followed
by a TEST of the input where only the Z flag is consumed, we can
replace it with the opposite check of the C flag.

We should be able to do the same for BLSMSK and BLSR, but the
naive test case for those is being optimized to a subo by
CodeGenPrepare.

Reviewers: spatel, RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63589

llvm-svn: 363957