[MLIR] Narrow to string comparisons; defer other ops to follow-ups

Scope this PR to the string comparison operators (==, !=, <, <=, >, >=) over
mlir_string / Masked(mlir_string) / string literals, with validity propagated
for masked operands. Drops (to follow-up PRs) the affix/search/contains and
character-class predicate ops and the pylibcudf char-flags accessor they needed.
Trivial MLIR type/const helpers remain inlined per prior review.

Author: brandon-b-miller