[RISCV] Teach DAG combine what bits Zbp instructions demanded from their inputs.

This teaches DAG combine that shift amount operands for grev, gorc
shfl, unshfl only read a few bits.

This also teaches DAG combine that grevw, gorcw, shflw, unshflw,
bcompressw, bdecompressw only consume the lower 32 bits of their
inputs.

In the future we can teach SimplifyDemandedBits to also propagate
demanded bits of the output to the inputs in some cases.