[X86] Require 32-byte alignment for 32-byte VMOVNTs.

[X86] Require 32-byte alignment for 32-byte VMOVNTs.

We used to accept (and even test, and generate) 16-byte alignment
for 32-byte nontemporal stores, but they require 32-byte alignment,
per SDM. Found by inspection.

Instead of hardcoding 16 in the patfrag, check for natural alignment.
Also fix the autoupgrade and the various tests.

Also, use explicit -mattr instead of -mcpu: I stared at the output
several minutes wondering why I get 2x movntps for the unaligned
case (which is the ideal output, but needs some work: see FIXME),
until I remembered corei7-avx implies +slow-unaligned-mem-32.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246733 91177308-0d34-0410-b5e6-96231b3b80d8