[RISCV] Improve constant materialization for cases that can use LUI+ADDI instead of LUI+ADDIW.

It's possible that we have a constant that isn't simm32 so we can't
use LUI+ADDIW, but we can use LUI+ADDI. Because ADDI uses a sign
extended constant, it's possible that after subtracting it out, we
end up with a simm32 that maps to LUI.

This patch detects this case after removing Lo12 and before shifting
the value for SLLI.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D124222