This patch fixes a bug occurring when properties are mixed with any of
the InferType traits, causing tblgen to crash. A simple reproducer is:
```
def _TypeInferredPropOp : NS_Op<"type_inferred_prop_op_with_properties", [
    AllTypesMatch<["value", "result"]>
  ]> {
  let arguments = (ins Property<"unsigned">:$prop, AnyType:$value);
  let results = (outs AnyType:$result);
  let hasCustomAssemblyFormat = 1;
}
```

The issue occurs because of the call:
```
op.getArgToOperandOrAttribute(infer.getIndex());
```
To understand better the issue, consider:
```
attrOrOperandMapping = [Operand0]
arguments = [Prop0, Operand0]
```
In this case, `infer.getIndex()` will return `1` for `Operand0`, but
`getArgToOperandOrAttribute` expects `0`, causing the discrepancy that
causes the crash.

The fix is to change `attrOrOperandMapping` to also include props.

This PR introduces the `vector.to_elements` op, which decomposes a
vector into its scalar elements. This operation is symmetrical to the
existing `vector.from_elements`.

Examples:

```
    // Decompose a 0-D vector.
    %0 = vector.to_elements %v0 : vector<f32>
    // %0 = %v0[0]

    // Decompose a 1-D vector.
    %0:2 = vector.to_elements %v1 : vector<2xf32>
    // %0#0 = %v1[0]
    // %0#1 = %v1[1]

    // Decompose a 2-D.
    %0:6 = vector.to_elements %v2 : vector<2x3xf32>
    // %0#0 = %v2[0, 0]
    // %0#1 = %v2[0, 1]
    // %0#2 = %v2[0, 2]
    // %0#3 = %v2[1, 0]
    // %0#4 = %v2[1, 1]
    // %0#5 = %v2[1, 2]
```

This op is aimed at reducing code size when modeling "structured" vector
extractions and simplifying canonicalizations of large sequences of
`vector.extract` and `vector.insert` ops into `vector.shuffle` and other
sophisticated ops that can re-arrange vector elements.

Rename `ListInit::getValues()` to `getElements()` to better match with
other `ListInit` members like `getElement`. Keep `getValues()` for
existing downstream code but mark it deprecated.

Trying to constrain two results to be of the same type using
`AllTypesMatch` would cause `mlir-tablgen` to crash on this
assertion[1].

Example:

```tblgen
def OpL5 : NS_Op<"op_with_same_but_unconstraint_results",
    [AllTypesMatch<["result_a", "result_b"]>]> {
  let results = (outs AnyType:$result_a, AnyType:$result_b);
}
```

This is because there was a small bug when constructing the `inferences`
graph from these constraints: The sources should be specified by the
combined arg/result index (in other words, with results negative) not
with the result index.


[1]
https://github.com/llvm/llvm-project/blob/99612a3a18e0c40aac9c52b68e67b106f97ed4fa/mlir/lib/TableGen/Operator.cpp#L526

Note that PointerUnion::{is,get} have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Eliminate `llvm::` namespace qualifier for commonly used types in MLIR
TableGen backends to reduce code clutter.

This reverts commit 0eed3055511381436ee69d1caf64a4af47f8d65c and applies
additional fixes in `verifyArgument` in OmpOpGen.cpp for gcc-7 bot
failures

Reverts llvm/llvm-project#112316

Bots are failing.

Use const pointers for various `Init` objects. This is a part of effort
to have better const correctness in TableGen backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089