[RISCV] Add SEW to RISCVInversePseudoTable

Now that scheduler resources are split by SEW for some instructions,
add the ability to map (BaseInstr, LMUL, SEW) -> Pseudo. For
BaseInstrs that are not split by SEW, 0 is the default key.

This does not change the size of the table since there was an 8
bit hole.

Differential Revision: https://reviews.llvm.org/D154136