[RISCV] Enable subregister liveness by default

This commit enable the subregister liveness by default in RISC-V.

It was previously disabled in https://reviews.llvm.org/D129646 after a previous attempt to enabled it https://reviews.llvm.org/D128016.

We believe that https://reviews.llvm.org/D129735 fixes the issue that caused it to be disabled.

Reviewed By: craig.topper, kito-cheng

Differential Revision: https://reviews.llvm.org/D145546