[DebugInfo][InstrRef] Add unit tests for transfer-function building

This patch adds some unit tests for the machine-location transfer-function
building parts of InstrRefBasedLDV: i.e., test that if we feed some MIR
into the transfer-function building code, does it create the correct
transfer function.

There are a number of minor defects that get corrected in the process:
 * The unit test was selecting the x86 (i.e. 32 bit) backend rather than
   x86_64's 64 bit backend,
 * COPY instructions weren't actually having their subregister values
   correctly represented in the transfer function. Subregisters were being
   defined by the COPY, rather than taking the value in the source register.
 * SP aliases were at risk of being clobbered, if an SP subregister was
   clobbered.

Differential Revision: https://reviews.llvm.org/D112006