When executed in the context of canonicalization, the folders are
invoked in a fixed-point iterative process. However in the context of an
API like `createOrFold()` or in DialectConversion for example, we expect
a "one-shot" call to fold to be as "folded" as possible. However, even
when folders themselves are indempotent, folders on a given operation
interact with each other. For example:

```
// X = 0 + Y
%X = arith.addi %c_0, %Y : i32
```

should fold to %Y, but the process actually involves first the folder
provided by the IsCommutative trait to move the constant to the right.
However this happens after attempting to fold the operation and the
operation folder isn't attempt again after applying the trait folder.

This commit makes sure we iterate until fixed point on folder
applications.

Fixes #159844

### Description

This patch improves the folding efficiency of `vector.insert` and
`vector.extract` operations by not returning early after successfully
converting dynamic indices to static indices.

This PR also renames the test pass `TestConstantFold` to
`TestSingleFold` and adds comprehensive documentation explaining the
single-pass folding behavior.

### Motivation

Since the `OpBuilder::createOrFold` function only calls `fold` **once**,
the current `fold` methods of `vector.insert` and `vector.extract` may
leave the op in a state that can be folded further. For example,
consider the following un-folded IR:
```
%v1 = vector.insert %e1, %v0 [0] : f32 into vector<128xf32>
%c0 = arith.constant 0 : index
%e2 = vector.extract %v1[%c0] : f32 from vector<128xf32>
```
If we use `createOrFold` to create the `vector.extract` op, then the
result will be:
```
%v1 = vector.insert %e1, %v0 [127] : f32 into vector<128xf32>
%e2 = vector.extract %v1[0] : f32 from vector<128xf32>
```
But this is not the optimal result. `createOrFold` should have returned
`%e1`.
The reason is that the execution of fold returns immediately after
`extractInsertFoldConstantOp`, causing subsequent folding logics to be
skipped.

---------

Co-authored-by: Yang Bai <yangb@nvidia.com>