[RISCV] Disable subregister liveness by default

We previously enabled subregister liveness by default when compiling
with RVV. This has been shown to cause miscompilations where RVV
register operand constraints are not met. A test was added for this in
D129639 which explains the issue in more detail.

Until this issue is fixed in some way, we should not be enabling
subregister liveness unless the user asks for it.

Reviewed By: craig.topper, rogfer01, kito-cheng

Differential Revision: https://reviews.llvm.org/D129646