[InstCombine][X86] Tweak generic expansion of PACKSS/PACKUS to shuffle then truncate. NFCI.
[InstCombine][X86] Tweak generic expansion of PACKSS/PACKUS to shuffle then truncate. NFCI.

This has no effect on constant folding but will be useful when we expand non-saturating PACKSS/PACKUS intrinsics.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@359191 91177308-0d34-0410-b5e6-96231b3b80d8

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