Roman Lebedev 62fc5f1640 [DAGCombiner] Add a most basic combineShuffleToZeroExtendVectorInReg() ...

Sometimes we end up with a shuffles in DAG that would be
better represented as a `ISD::ZERO_EXTEND_VECTOR_INREG`,
and a failure to do so causes suboptimal codegen in a number of cases,
especially when we will then cast vector to scalar.

I acknowledge, the test changes here are rather underwhelming,
but as with all of codegen, it's always a yak shawing,
and this is the most stripped down version of the patch
that shows *some* effect without having insurmountable amount
of fallout to deal with. The next change resolves this regression.

The transformation will be extended in follow-ups.

2022-12-26 22:54:03 +03:00

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