[Matrix] Fix miscompile for NT matmul if the transpose has other use

We should only add the fake lowering entry for the matrix remark if the
transpose is not lowered on its own.  `MapVector::insert` is used to insert
the entry during proper lowering which does not overwrite the fake entry in
the map.

We actually had test coverage for this but the reference output code was
wrong; it was storing undef rather than the transposed column.

Also add an assert that would have caught this.

Differential Revision: https://reviews.llvm.org/D106457