[LV][REVEC][AArch64] Proof of concept for re-vectorisation
This shows the changes required to enable basic re-vectorisation support in LoopVectorizer. Most of the diff comes from the added tests, the changes to LoopVectorizer files are rather minimal. This proof-of-concept has obvious limitations and only represents the first building block.
My hope is that this helps discussions and complements the RFC at https://discourse.llvm.org/t/rfc-re-vectorisation-to-wider-vectors-in-loopvectorizer/91071.
Support for re-vectorisation is hidden behind a -vectorize-vector-loops flag and LV will bail out if it encounters constructs that are not yet supported. For example:
- shufflevectors
- gather/scatter and interleaved accesses
- target intrinsics
- reductions
- if-conversion or tail folding
