[X86] Allow pack instructions to be used for 512->256 truncates when...

[X86] Allow pack instructions to be used for 512->256 truncates when -mprefer-vector-width=256 is causing 512-bit vectors to be split

If we're splitting the 512-bit vector anyway and we have zero/sign bits, then we might as well use pack instructions to concat and truncate at once.

Differential Revision: https://reviews.llvm.org/D65904

llvm-svn: 368210