[AVX512] Use X86VectorVTInfo in the masking helper classes and the FMAs

No functionality change.

Makes the code more compact (see the FMA part).

This needs a new type attribute MemOpFrag in X86VectorVTInfo.  For now I only
defined this in the simple cases.  See the commment before the attribute.

Diff of X86.td.expanded before and after is empty except for the appearance of
the new attribute.