IR with matrix intrinsics is likely to also contain large vector
operations, which can benefit from early simplifications.
This is the last step in a series of changes to improve code-gen for
code using matrix subscript operators with the C/C++ matrix extension in
CLang, like
using matrix_t = double __attribute__((matrix_type(15, 15)));
void foo(unsigned i, matrix_t &A, matrix_t &B) {
for (unsigned j = 0; j < 4; ++j)
for (unsigned k = 0; k < i; k++)
B[k][j] -= A[k][j] * B[i][j];
}