Split the FindLastIV RecurKind into SMax and UMax variants, depending on
the reduction op produced.

Having FindFirstIV tests in if-reduction.ll is misleading, and
iv-select-cmp.ll is already too large.

Follow-up to dfca6c0d3bf9d1a056 to extend isUnrolled handle any unrolled
VPlan, which means there's a single UF, but it will be > 1 if unrolling
took place.

After unrolling, there may be additional simplifications that can be
applied. One example is removing SCALAR-STEPS for the first part where
only the first lane is demanded.

This removes redundant adds of 0 from a large number of tests (~200),
many which I am still working on updating.

In preparation for removing redundant WideIV steps added in
https://github.com/llvm/llvm-project/pull/119284.

PR: https://github.com/llvm/llvm-project/pull/123655

Consider the following loop:
```
  int rdx = init;
  for (int i = 0; i < n; ++i)
    rdx = (a[i] > b[i]) ? i : rdx;
```
We can vectorize this loop if `i` is an increasing induction variable.
The final reduced value will be the maximum of `i` that the condition
`a[i] > b[i]` is satisfied, or the start value `init`.

This patch added new RecurKind enums - IFindLastIV and FFindLastIV.

---------

Co-authored-by: Alexey Bataev <5361294+alexey-bataev@users.noreply.github.com>

Pre-commit for #67812

The vectorization of the FindLastIV reduction does not depend on the
nocapture and readonly attributes.

To help future contributors understand a couple of mysterious
out-of-bound tests, add a brief comment to each.

The most straightforward extension to D150851 would involve handling the
decreasing IV case, for which tests have been added in 110ec1863a
(LoopVectorize/iv-select-cmp: add test for decreasing IV, const start).
However, the commit missed a testcase for the out-of-bound sentinel
value LONG_MAX, which should not be vectorized. Fix this by adding a
test corresponding to the following program:

  long test(long *a) {
    long rdx = 331;
    for (long i = LONG_MAX; i >= 0; i--) {
      if (a[i] > 3)
        rdx = i;
    }
    return rdx;
  }

Differential Revision: https://reviews.llvm.org/D157969