[NFC][RISCV] Remove redundant pseudo instructions for vector load/store.

Not all combinations of SEW and LMUL we need to support. For example, we
only need to support [M1, M2, M4, M8] for SEW = 64. There is no need to
define pseudos for PseudoVLSE64MF8, PseudoVLSE64MF4, and PseudoVLSE64MF2.

Differential Revision: https://reviews.llvm.org/D95667