[X86] Improve the type checking in isLegalMaskedLoad and isLegalMaskedGather.

We were just checking pointer size and type primitive size. But this caused unintended things like vectors of half being accepted by masked load/store.

For FP we now explicitly check for only double and float.

For pointers we now let any pointer through. Trusting that only 32 and 64 would be used to generate assembly.

We only check bitwidth after checking that the type is an integer.

llvm-svn: 355667