[RISCV] Enable subregister liveness tracking for RVV.

RVV makes heavy use of subregisters due to LMUL>1 and segment
load/store tuples. Enabling subregister liveness tracking improves the quality
of the register allocation.

I've added a command line that can be used to turn it off if it causes compile
time or functional issues. I used the command line to keep the old behavior
for one interesting test case that was testing register allocation.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D125108