[RISCV] Fix a few corner case bugs in RISCVMergeBaseOffsetOpt::matchLargeOffset

The immediate for LUI is stored as 20-bit unsigned value. We need
to sign extend if after shifting by 12 to match the instruction
behavior.

If we find an LUI+ADDI on RV64, it means the constant isn't a
simm32. If it was, we would have emitted LUI+ADDIW from constant
materialization. Make sure the constant is a simm32 before folding.
This appears to match gcc.

A future patch will add support for LUI+ADDIW on RV64.