[Matrix] Use ArrayType for allocas instead of VectorType.

When creating an alloca to copy a matrix due to memory conflicts, those
allocas used to use VectorTypes, which forced them to have huge
alignments for large vectors.

This patch updates LowerMatrixIntrinsics to use a corresponding array
type, like Clang already does, to get more manageable alignments.

Reviewed By: anemet, thegameg

Differential Revision: https://reviews.llvm.org/D118239