[RISCV] Enable (srl (and X, C2), C) to form SRLIW in more cases.

Don't require the AND has one use and don't depend on
targetShrinkDemandedConstant turning C2 into 0xffffffff. Instead,
check that the constant is 0xffffffff after replacing any bits
that will be shifted out with 1s.

Another way to fix this might be to prevent SimplifyDemandedBits
from destroying the ANDI after type legalization using
targetShrinkDemandedBits. That would prevent the CSE that created
this mess. targetShrinkDemandedBits is currently only enable after
legalize ops. Quick experiment shows we can't just change when it
runs, we would need to try a different heuristic for post type
legalization.