[RISCV] Use XORI/SLLI/ADDI to when materializing select of constants (#155845)
This case is the inverse of the one introduced in #155644. The
complexity with the inversion is that we need to also invert the
condition before shifting it. I had originally planned to only do so
when the condition was "cheaply" invertible (i.e. didn't require the
xori), but when looking more closely at the diffs I noticed that while
the XORI prevents this from being an icount improvement, and actually
lengthens slightly the critical path, it does still reduce the number of
registers needed.