[ARM] mark tMOVi32imm as killing flags

Mark the tMOVi32imm pseudo instr as killing the flags register.

The pseudo instruction expands to a sequence of 7 movs/lsls/adds
instructions, which are all Thumb-1 flag setting instructions.

For a test case, take an existing arm test which checks for
"Don't CSE a cmp across a call that clobbers CPSR."
and retarget it at thumbv6m execute-only.

Reviewed By: stuij

Differential Revision: https://reviews.llvm.org/D154845

Change-Id: I8f8209fbc40a833f8875629937b9606c1e2c021d