[llvm] [VectorCombine] foldShuffleToIdentity - handle bitcasts with equal element counts (PR #97731)

[via llvm-commits]

bgra8 wrote:

@RKSimon early heads up that this is causing test failures for open source https://github.com/google/highway for test `hwy/tests/sums_abs_diff_test.cc`

We are working on a reproducer, a revert would be great in the mean time.

https://github.com/llvm/llvm-project/pull/97731