[X86] Don't mark SEXTLOAD from v4i8/v4i16/v8i8 as Custom on pre-sse4.1.

This seems to be getting in the way more than its helping. This does mean we stop scalarizing some cases, but I'm not convinced the scalarization was really better.

Some of the changes to vsel-cmp-load.ll are a regression but D56156 should fix it.

llvm-svn: 350159