[X86][AVX2] Improve lowerShuffleAsRepeatedMaskAndLanePermute permutation of 64-bit...

[X86][AVX2] Improve lowerShuffleAsRepeatedMaskAndLanePermute permutation of 64-bit sub-lanes

As discussed on PR28136, lowerShuffleAsRepeatedMaskAndLanePermute was attempting to match repeated masks at the 128-bit level and then permute the resultant lanes at the 128-bit (AVX1) or 64-bit (AVX2) sub-lane level.

This change allows us to create the repeated masks at the sub-lane level (and then concat them together to create a 128-bit repeated mask) and then select which sub-lane to permute. This has no effect on the AVX1 codegen.

Fixes PR28136.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275543 91177308-0d34-0410-b5e6-96231b3b80d8