[Passes] Run vector-combine early with -fenable-matrix.

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];
    }

https://clang.godbolt.org/z/6dKxK1Ed7

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D102496