[X86] Remove most of the SEXTLOAD Custom setOperationAction calls under...

[X86] Remove most of the SEXTLOAD Custom setOperationAction calls under -x86-experimental-vector-widening-legalization.

Leave just the v4i8->v4i64 and v8i8->v8i64, but only enable them on pre-sse4.1 targets when 64-bit mode is enabled. In those cases we end up creating sext loads that get scalarized to code that looks better than what we get from loading into a vector register and doing a multiple step sign extend using unpacks and shifts.

llvm-svn: 347180