[mlir] tighten LLVM dialect verifiers to generate valid LLVM IR

Verification of the LLVM IR produced when translating various MLIR dialects was
only active when calling the translation programmatically. This has led to
several cases of invalid LLVM IR being generated that could not be caught with
textual mlir-translate tests. Add verifiers for these cases and fix the tests
in preparation for enforcing the validation of LLVM IR.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D96774