Move to RISCVInstrInfo since we need RISCVSubtarget now.
Instead of asking if only the lower 32 bits are used we can now
ask if the lower N bits are used. This will be needed by a future
patch.
SLLI and ADD are more compressible than SLLIW and ADDW. SLLI/ADD both have a 5-bit register encoding. SLLIW/ADDW have a 3-bit register encoding. They both require the dest to also be one of the sources.
We aggressively form ADDW/SLLIW as it helps hasAllWBitUsers in RISCVISelDAGToDAG to not require recursion. So we need a pass to remove excessive -w suffixes.
Differential Revision: https://reviews.llvm.org/D139948
