Decouple LLVM dialect from Standard dialect

Due to the absence of ODS support for enum attributes, the implementation of
the LLVM dialect `icmp` operation was reusing the comparison predicate from the
Standard dialect, creating an avoidable library dependency.  With ODS support
and ICmpPredicate attribute recently introduced, the dependency is no longer
justified.  Update the Standard to LLVM convresion to also convert the
CmpIPredicate into LLVM::ICmpPredicate and remove the unnecessary includes.

Note that the MLIRLLVMIR library did not explicitly depend on MLIRStandardOps,
requiring dependees of MLIRLLVMIR to also depend on MLIRStandardOps, which
should no longer be the case.

PiperOrigin-RevId: 258148456