[X86] Add a test to reveal a bug in CMOV conversion.

CMOV conversion first rewrites all CMOVs with memory load to branches.
Then runs a second pass to convert other CMOVs in loops if profitable.
But the first pass doesn't add new basic blocks to MachineLoopInfo,
CMOVs in these blocks are ignored in the subsequent pass.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D104692