[VectorCombine] support mismatching extract/insert indices for foldInsExtFNeg (#126408)
insertelt DestVec, (fneg (extractelt SrcVec, Index)), Index
-> shuffle DestVec, (shuffle (fneg SrcVec), poison, SrcMask), Mask
In previous, the above transform was only possible if the Extract/Insert
Index was the same; this patch makes the above transform possible
even if the two indexes are different.
Proof: https://alive2.llvm.org/ce/z/aDfdyG
Fixes: https://github.com/llvm/llvm-project/issues/125675