[X86] Add AVX 256-bit load and stores to fast isel.

I'm not sure why this was missing for so long.

This also exposed that we were picking floating point 256-bit VMOVNTPS for some integer types in normal isel for AVX1 even though VMOVNTDQ is available. In practice it doesn't matter due to the execution dependency fix pass, but it required extra isel patterns. Fixing that in a follow up commit.

llvm-svn: 271481