[mlir][llvm] Fix verifier for const int and dense (#74340)

Continuation of https://github.com/llvm/llvm-project/pull/74247 to fix
https://github.com/llvm/llvm-project/issues/56962. Fixes verifier for
(Integer Attr):
```mlir
llvm.mlir.constant(1 : index) : f32
```
and (Dense Attr):
```mlir
llvm.mlir.constant(dense<100.0> : vector<1xf64>) : f32
```

## Integer Attr

The addition that this PR makes to `LLVM::ConstantOp::verify` is meant
to be exactly verifying the code in
`mlir::LLVM::detail::getLLVMConstant`:


https://github.com/llvm/llvm-project/blob/9f78edbd20ed922cced9482f7791deb9899a6d82/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp#L350-L353

One failure mode is when the `type` (`llvm.mlir.constant(<value>) :
<type>`) is not an `Integer`, because then the `cast` in
`getIntegerBitWidth` will crash:


https://github.com/llvm/llvm-project/blob/dca432cb7b1c282f5dc861095813c4f40f109619/llvm/include/llvm/IR/DerivedTypes.h#L97-L99

So that's now caught in the verifier.

Apart from that, I don't see anything we could check for. `sextOrTrunc`
means "Sign extend or truncate to width" and that one is quite
permissive. For example, the following doesn't have to be caught in the
verifier as it doesn't crash during `mlir-translate -mlir-to-llvmir`:

```mlir
llvm.func @main() -> f32 {
  %cst = llvm.mlir.constant(100 : i64) : f32
  llvm.return %cst : f32
}
```

## Dense Attr

Crash if not either a MLIR Vector type or one of these:


https://github.com/llvm/llvm-project/blob/9f78edbd20ed922cced9482f7791deb9899a6d82/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp#L375-L391