[llvm] [SelectionDAG][AArch64][X86] Don't scalarize vector smul.fix.sat/umul.fix.sat (PR #209351)

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guyfischman wrote:

> Please reply to this message to confirm that you have read these policies, especially the LLVM AI Tool Use Policy, and that any AI tool usage has been noted in the PR description.

I confirm that I've read the policies and noted AI use in the PR description.

https://github.com/llvm/llvm-project/pull/209351