[X86][SSE] Truncate with PACKSS any input with sufficient sign-bits

So far we've only been using PACKSS truncations with 'all-bits or zero-bits' patterns (vector comparison results etc.). When really we can safely use it for any case as long as the number of sign bits reach down to the last 16-bits (or 8-bits if we're truncating to bytes).

The next steps after this is add the equivalent support for PACKUS and to support packing to sub-128 bit vectors for truncating stores etc.

Differential Revision: https://reviews.llvm.org/D39476

llvm-svn: 317086