This adds support for unary operands, and unary + ternary intrinsics in
scalarizeOpOrCmp (FKA scalarizeBinOpOrCmp).
The motivation behind this is to scalarize more intrinsics in
VectorCombine rather than in DAGCombine, so we can sink splats across
basic blocks: see https://github.com/llvm/llvm-project/pull/137786
The main change required is to generalize the existing VecC0/VecC1 rules
across n-ary ops:
- An operand can either be a constant vector or an insert of a scalar
into a constant vector
- If it's an insert, the index needs to be static and in bounds
- If it's an insert, all indices need to be the same across all operands
- If all the operands are constant vectors, bail as it will get constant
folded anyway
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