[X86] When creating X86ISD::MGATHER nodes from AVX2 gather intrinsics, cast the mask...

[X86] When creating X86ISD::MGATHER nodes from AVX2 gather intrinsics, cast the mask to integer type.

The gather intrinsics use a floating point mask when the result
type is FP. But we call DemandedBits on the mask assuming its an
integer type. We also use integer types when we create it from
generic IR. So add a bitcast to the intrinsic path to guarantee
the integer type.