[Matrix] Use alignment info when lowering loads/stores.

[Matrix] Use alignment info when lowering loads/stores.

This patch updates LowerMatrixIntrinsics to preserve the alignment
specified at the original load/stores and the align attribute for the
pointer argument of the column.major.load/store intrinsics.

We can always use the specified alignment for the load of the first
column. For subsequent columns, the alignment may need to be reduced.

For ConstantInt strides, compute the offset for the start of the column in
bytes and use commonAlignment to get the largest valid alignment.

For non-ConstantInt strides, we need to take the common alignment of the
initial alignment and the element size in bytes.

Reviewers: anemet, Gerolf, hfinkel, andrew.w.kaylor, LuoYuanke, rjmccall

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D81960