[RISCV] Use selectImm for RV32. NFC

Previously we used selectImm for RV64 and isel patterns for
RV32. This should be NFC, but will allow RV32 and RV64 to share
improvements in the future. For example, it might be useful to
use BSETI from Zbs to make single bit constants.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D98877