This refactors fixSuccessorPhis from
LoopIdiomVectorize::transformByteCompare and uses it in
LoopIdiomVectorize::expandFindFirstByte to ensure that all successor
Phis have incoming values from the vector basic blocks.

Fixes #156588.

---------

Co-authored-by: Ricardo Jesus <rjj@nvidia.com>

To match the general direction of the loop vectorizer and reduce our
dependency on [trivial VP
intrinsics](https://discourse.llvm.org/t/rfc-remove-codegen-support-for-trivial-vp-intrinsics-in-the-risc-v-backend/87999),
replace the use of vp.icmp with a regular icmp. RISCVVLOptimizer can
take care of propagating VL.

CreateVScale took a scaling parameter that had a single use outside of
IRBuilder with all other callers having to create a redundant
ConstantInt. To work round this some code perferred to use
CreateIntrinsic directly.

This patch simplifies CreateVScale to return a call to the llvm.vscale()
intrinsic and nothing more. As well as simplifying the existing call
sites I've also migrated the uses of CreateIntrinsic.

Whilst IRBuilder used CreateVScale's scaling parameter as part of the
implementations of CreateElementCount and CreateTypeSize, I have
follow-on work to switch them to the NUW varaiety and thus they would
stop using CreateVScale's scaling as well. To prepare for this I have
moved the multiplication and constant folding into the implementations
of CreateElementCount and CreateTypeSize.

As a final step I have replaced some callers of CreateVScale with
CreateElementCount where it's clear from the code they wanted the
latter.

Most callers want a constant index. Instead of making every caller
create a ConstantInt, we can do it in IRBuilder. This is similar to
createInsertElement/createExtractElement.

This patch adds a new loop to LoopIdiomVectorizePass, enabling it to
recognise and vectorise loops such as:
```cpp
template<class InputIt, class ForwardIt>
InputIt find_first_of(InputIt first, InputIt last,
                      ForwardIt s_first, ForwardIt s_last)
{
  for (; first != last; ++first)
    for (ForwardIt it = s_first; it != s_last; ++it)
      if (*first == *it)
        return first;
  return last;
}
```

These loops match the C++ standard library function `std::find_first_of`.

The LoopIdiomVectorize pass already creates calls to the intrinsic
experimental_cttz_elts, but PR #88385 will start calling this more
too so I've created a helper for it.

Compile-time improvement:
http://llvm-compile-time-tracker.com/compare.php?from=13996378d81c8fa9a364aeaafd7382abbc1db83a&to=861ffa4ec5f7bde5a194a7715593a1b5359eb581&stat=instructions:u
baseline: 803eaf29267c6aae9162d1a83a4a2ae508b440d3
```
Top 5 improvements:
  stockfish/movegen.ll 2541620819 2538599412 -0.12%
  minetest/profiler.cpp.ll 431724935 431246500 -0.11%
  abc/luckySwap.c.ll 581173720 580581935 -0.10%
  abc/kitTruth.c.ll 2521936288 2519445570 -0.10%
  abc/extraUtilTruth.c.ll 1216674614 1215495502 -0.10%
Top 5 regressions:
  openssl/libcrypto-shlib-sm4.ll 1155054721 1155943201 +0.08%
  openssl/libcrypto-lib-sm4.ll 1155054838 1155943063 +0.08%
  spike/vsm4r_vv.ll 1296430080 1297039258 +0.05%
  spike/vsm4r_vs.ll 1312496906 1313093460 +0.05%
  nuttx/lib_rand48.c.ll 126201233 126246692 +0.04%
Overall: -0.02112308%
```

Teach LoopIdiomVectorize to use VP intrinsics to replace the byte
compare loops. Right now only RISC-V uses LoopIdiomVectorize of this
style.

To pave the way for porting LIV to RISC-V, which uses VP intrinsics for
vectors.

NFC.

This is a helper to avoid writing `getModule()->getDataLayout()`. I
regularly try to use this method only to remember it doesn't exist...

`getModule()->getDataLayout()` is also a common (the most common?)
reason why code has to include the Module.h header.