[llvm] [X86] Attempt to canonicalize vXf64 SHUFPD shuffle masks with undef elts to improve further folding (PR #116419)

[Mikhail Goncharov via llvm-commits]

metaflow wrote:

@RKSimon here is a IR from the test
[stage_strided_loads.ll.txt](https://github.com/user-attachments/files/17832335/stage_strided_loads.ll.txt)
`llc -- -march=x86-64 -o /tmp/out.s stage_strided_loads.ll.txt` fails for me

https://github.com/llvm/llvm-project/pull/116419