[RISCV] Add test cases that show failure to use some W instructions if they...

[RISCV] Add test cases that show failure to use some W instructions if they are proceeded by a load. NFC

The loads end up becoming sextload/zextload which prevent our
isel patterns from finding the sign_extend_inreg or AND instruction
we need.

The easiest way to fix this is to use computeKnownBits or
ComputeNumSignBits in our isel matching to catch this.