[InstCombine][X86] Use generic expansion of PACKSS/PACKUS for constant folding. NFCI.

This patch rewrites the existing PACKSS/PACKUS constant folding code to expand as a generic expansion.

This is a first NFCI step toward expanding PACKSS/PACKUS intrinsics which are acting as non-saturating truncations (although technically the expansion could be used in all cases - but we'll probably want to be conservative).

llvm-svn: 359111