[X86] Add some custom type legalization rules for truncate with -x86-experimental...

[X86] Add some custom type legalization rules for truncate with -x86-experimental-vector-widening-legalization.

This avoids some nasty shuffles when we have avx512. It will also prevent using zmm truncate instructions when a ymm instruction that zeroes part of an xmm register will do. Also avoid using avx512 truncate instructions when the input is 128 bits or less. These instructions are 2 uops on skx so we can probably find a better single uop shuffle like pshufb.

llvm-svn: 346936