[DebugInfo][InstrRef] Instrument x86 CMOV conversion to preserve variable values

X86's CMOV conversion transforms CMOV instructions into control flow between
blocks, meaning the value is computed by a PHI rather than a "real" machine
instruction. In instruction-referencing mode, we need to transfer the
instruction label between the old CMOV and the new PHI instruction to mark
where the variable value is computed.

There's an extra complication in that memory operands can be unfolded from the
CMOV and sunk into the new blocks -- the test checks both scenarios where the
instruction number has to hop between instructions.

This omission exposed by Dexter testing.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D145565