[llvm] [InstCombine] Do not combine shuffle+bitcast if the bitcast is eliminable. (PR #135769)

[Ricardo Jesus via llvm-commits]

rj-jesus wrote:

Hi @nikic - Would it be reasonable to backport this to LLVM 20?
This accounts for a ~30% regression in the affected workloads where this was detected.

https://github.com/llvm/llvm-project/pull/135769