[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346936 91177308-0d34-0410-b5e6-96231b3b80d8