Update LLVM Dialect documentation

Addressing post-submit comments.  The `getelementptr` operation now supports
non-constant indexes, similarly to LLVM, and this functionality is exercised by
the lowering to the dialect.  Update the documentation accordingly.

List the values of integer comparison predicates, which currently correspond to
those of CmpIOp in MLIR.  Ideally, we would use strings instead, but it
requires additional support for argument conversion in both the dialect
lowering pass and the LLVM translator.

PiperOrigin-RevId: 235678877