[AVX-512] Fix masked VPERMI2PS isel when the index comes from a bitcast.

We need to bitcast the index operand to a floating point type so that it matches the result type. If not then the passthru part of the DAG will be a bitcast from the index's original type to the destination type. This makes it very difficult to match. The other option would be to add 5 sets of patterns for every other possible type.

llvm-svn: 280696