[llvm] [InstCombine] Compact shuffle operands by eliminating unused elements (PR #175255)

[Nikita Popov via llvm-commits]

nikic wrote:

Pretty sure this needs to go in VectorCombine. Supported shuffles are target specific, and "simplifying" to a non-native shuffle can be very expensive.

https://github.com/llvm/llvm-project/pull/175255