[X86] Prevent DAG combine from folding a bitcast from vXi1 to iX with a store...

[X86] Prevent DAG combine from folding a bitcast from vXi1 to iX with a store on pre-AVX512 targets.

If we fold the bitcast into the store we'll end up creating a truncating store to vXi1 that will get scalarized. Instead allow the bitcast to be turned into a movmsk.

We probably need to do something if the store itself is a vXi1 type, but I'll leave that til a testcase appears.

llvm-svn: 347632