[VectorCombine] Expand Intrinsic::vector_insert calls into shufflevectors

[RKSimon]

These are being generated by SLP and, the expansion is already performed in InstCombinerImpl.visitCallInst - but this occurs AFTER vector-combine has run, so further cost driven vector-combine shuffle folds are missed.

[llvmbot]

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

This is already performed in InstCombinerImpl.visitCallInst - but this occurs AFTER vector-combine has run, so further cost driven vector-combine shuffle folds are missed.

We should only need to handle vector_insert calls with FixedVectorTypes and there might not be any reason for a cost check, but we must ensure the shuffle are added back to the worklist to allow further expansion.

[laurenmchin]

hello, i'd like to take this on. could i please have this assigned to me?