[X86] NFC: expand inline memcmp test coverage

1) Adds SSE4.1 coverage.
2) Adds prefer-256-bit or not coverage.
3) Adds more power-of-two tests up to 512 bytes.
4) Adds power-of-two-minus-one tests to verify overlapping loads.
5) Adds power-of-two-plus-one-half tests (48, 96, 192, and 384).
6) Adds greater-than/less-than tests from 16 to 512 bytes.

https://reviews.llvm.org/D69222