[RISCV] Support isel of scalable vector bitcasts

These should be NOPs so we can just replace with the input. This
matches what SVE does with isel patterns for all permutations.
Custom isel saves us from having to list all permurations for
all LMULs.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D96921