[RISCV] Add (Proposed) Assembler Extend Pseudo-Instructions

There is an in-progress proposal for the following pseudo-instructions
in the assembler, to complement the existing `sext.w` rv64i instruction:
- sext.b
- sext.h
- zext.b
- zext.h
- zext.w

The `.b` and `.h` variants are available with rv32i and rv64i, and `zext.w` is
only available with `rv64i`.

These are implemented primarily as pseudo-instructions, as these instructions
expand to multiple real instructions. In the case of `zext.b`, this expands to a
single rv32/64i instruction, so it is implemented with an InstAlias (like
`sext.w` is on rv64i).

The proposal is available here: https://github.com/riscv/riscv-asm-manual/pull/61

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D92793