[LegalizeType] When LegalizeType procedure widens a masked_gather, set...

[LegalizeType] When LegalizeType procedure widens a masked_gather, set MemoryType's EltNum equal to Result's EltNum

When LegalizeType procedure widens a masked_gather, set MemoryType's EltNum equal to Result's EltNum.

As I mentioned in https://reviews.llvm.org/D91092, in previous code, If we have a v17i32's masked_gather in avx512, we widen it to a v32i32's masked_gather with a v17i32's MemoryType. When the SplitVecRes_MGATHER process this v32i32's masked_gather, GetSplitDestVTs will assert fail since what you are going to split is v17i32.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D93610