This attempts to standardize and extend some of the insert vector
element lowering. Most notably:
- More types are handled by splitting illegal vectors.
- The index type for G_INSERT_VECTOR_ELT is canonicalized to
TLI.getVectorIdxTy(), similar to extact_vector_element.
- Some of the existing patterns now have the index type specified to
make sure they can apply to GISel too.
- The C++ selection code has been removed, relying on tablegen patterns.
- G_INSERT_VECTOR_ELT with small GPR input elements are pre-selected to
use a i32 type, allowing the existing patterns to apply.
- Variable index inserts are lowered in post-legalizer lowering,
expanding into a stack store and reload.
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