[DAG] visitVECTOR_SHUFFLE - MergeInnerShuffle - improve shuffle(shuffle(x,y),shuffle...

[DAG] visitVECTOR_SHUFFLE - MergeInnerShuffle - improve shuffle(shuffle(x,y),shuffle(x,y)) merging

MergeInnerShuffle currently attempts to merge shuffle(shuffle(x,y),z) patterns into a single shuffle, using 1 or 2 of the x,y,z ops.

However if we already match 2 ops we might be able to handle the third op if its also a shuffle that references one of the previous ops, allowing us to handle some cases like:

shuffle(shuffle(x,y),shuffle(x,y))
shuffle(shuffle(shuffle(x,z),y),z)
shuffle(shuffle(x,shuffle(x,y)),z)
etc.

This isn't an exhaustive match and is dependent on the order the candidate ops are encountered - if one of the matched ops was a shuffle that was peek-able we don't go back and try to split that, I haven't found much need for that amount of analysis yet.

This is a preliminary patch that will allow us to later improve x86 HADD/HSUB matching - but needs to be reviewed separately as its in generic code and affects existing Thumb2 tests.

Differential Revision: https://reviews.llvm.org/D94671