[flang][hlfir] allow recursive intrinsic lowering

We need to allow recursive application of intrinsic lowering patterns,
otherwise we cannot lower nested calls of the same intrinsic e.g.
matmul(matmul(a, b), c).

matmul(matmul(a, b), matmul(c, d)) requires hlfir.associate of hlfir
expr with more than one use (TODO).

Differential Revision: https://reviews.llvm.org/D152284