[x86] use hasAVX2() rather than hasInt256(); NFC

[x86] use hasAVX2() rather than hasInt256(); NFC

These are aliases, but the thing we're checking here is that the target has
vpsllv*, not that the data type is 256-bit. Those instructions exist for
128-bit vectors too...but sadly, not for all element sizes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320170 91177308-0d34-0410-b5e6-96231b3b80d8