[LV] Use frozen start value for FindLastIV if needed. (#132691)
FindLastIV introduces multiple uses of the start value, where in the
original source there was only a single use, when the epilogue is
vectorized.
Each use of undef may produce a different result, so introducing
multiple uses can produce incorrect results when the input is
undef/poison.
If the start value may be undef or poison, freeze it and use the frozen
value, which will be the same at all uses.
See the following scenarios in Alive2:
* Both main and epilogue vector loops execute, go to exit block: https://alive2.llvm.org/ce/z/_TSvRr
* Both main and epilogue vector loops execute, go to scalar loop: https://alive2.llvm.org/ce/z/CsPj5v
* Only epilogue vector loop executes, go to exit block: https://alive2.llvm.org/ce/z/5XqkNV
* Only epilogue vector loop executes, go to scalar loop: https://alive2.llvm.org/ce/z/JUpqRN
The latter 2 show requiring freezing the resume phi. That means we cannot freeze
in the preheader. We could move the freeze to the main iteration count check, but
that would be a bit fragile to find and other transforms can sink the freeze if needed.
Depends on https://github.com/llvm/llvm-project/pull/132689
and https://github.com/llvm/llvm-project/pull/132690.
Fixes https://github.com/llvm/llvm-project/issues/126836
PR: https://github.com/llvm/llvm-project/pull/132691
2bdc1a1337