[RISCV] Prefer ADDI over ORI if the known bits are disjoint.

There is no compressed form of ORI but there is a compressed form
for ADDI.

This also works for XORI since DAGCombine will turn Xor with disjoint
bits in Or.

Note: The compressed forms require a simm6 immediate, but I'm doing
this for the full simm12 range.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D140674