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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@metaflow I can put in a stopgap fix but I'd appreciate a reduced repro - thanks.

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