[RISCV] Start merging demanded reasoning - starting with load/stores [nfc]

This change merges the logic for reasoning about demanded portions of the VTYPE register between the main dataflow algorithm and the backwards mutation post pass. In the process, we get to delete a bunch of now redundant code.

This should be entirely NFC. I included a slight hack (see TODO) to avoid changing behavior in the post pass while being able to use the generalized logic in the prepass. I will fix the TODO in a separate change once this lands.

Differential Revision: https://reviews.llvm.org/D127983