[RISCV][NFC] Remove classes/multiclasses for SEW-aware instructions

Instead, we add an argument `isSEWAware` to indicate that the
instruction is SEW-aware. Actually, the only difference is the
name of pseudo instructions.

And we remove postfix `_E` for all classes/multiclasses and remove
argument `sew` since it can be calculated from `log2sew`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D152428