[RISCV] Add tests for memcmp expansion

We add tests for the following cases:
* Length = 0, 1, 2, 3, 4, 5, 6, 7, 8, 15, 16, 31, 32, 63, 64, 127,
  128, runtime.
* Comparisons against zero.
* RUN lines for scalar/vector w/ or w/o strict align.
* Optimize for size.

Reviewers: topperc, preames

Reviewed By: topperc, preames

Pull Request: llvm#107824

Author: wangpc-pp