[HACK] X86: Disable isCopyInstrImpl for undef subregister defs

This is a workaround for a coalescer bug where coalescing
SUBREG_TO_REG ends up losing the liveness of the high bits of the
source register. The result is an incorrect undef subregister def
instead of preserving the high values. Work around the observed
failure after the resulting mov is eliminated during allocation until
a proper fix is ready. I believe the proper fix is to make
SUBREG_TO_REG use a tied operand.

The test should catch a regression originally observed after
b7836d85 and should not show a
difference after a496c8be is reverted.

https://reviews.llvm.org/D156164