[clang] [llvm] [AggressiveInstCombine] Shrink loads used in shufflevector rebroadcasts. (PR #128938)

[Ulrich Weigand via cfe-commits]

uweigand wrote:

In the SystemZ tests, the variables involved have been deliberated marked `volatile` to prevent this type of optimization (the tests are supposed to verify the basic operation of the intrinsics).  Does this new transformation respect `volatile`?

https://github.com/llvm/llvm-project/pull/128938