[X86] Update LiveVariables in more cases in convertToThreeAddress

Following on from D129634, this patch fixes more X86 CodeGen test
failures with D129213 applied, which adds verification of LiveIntervals
after the TwoAddressInstruction pass runs. These failures only showed up
with LLVM_ENABLE_EXPENSIVE_CHECKS=ON which adds the equivalent of an
implicit -verify-machineinstrs on all tests.

Differential Revision: https://reviews.llvm.org/D136596