[LV] Support argmin/argmax with strict predicates. (#170223)
Extend handleMultiUseReductions to support strict predicates (>, <),
matching the first index instead of the last for non-strict predicates.
Builds on top of https://github.com/llvm/llvm-project/pull/141431.
FindLast reductions with strict predicates are adjusted to compute the
correct result as follows:
1. Find the first canonical indices corresponding to partial min/max
values, using loop reductions.
2. Find which of the partial min/max values are equal to the overall
min/max value.
3. Select among the canonical indices those corresponding to the overall
min/max value.
4. Find the first canonical index of overall min/max and scale it back to
the original IV using VPDerivedIVRecipe.
5. If the overall min/max equals the starting min/max, the condition in
the loop was always false, due to being strict; return the original start
value in that case.
llvm-project
4042975b - [LV] Support argmin/argmax with strict predicates. (#170223)
