[FIRRTL][InferWidths] Make checkCycles Iterative

The method `InferWidths::checkCycles` is currently recursive. If `firtool` is spawned in a thread which has an abnormally low stack size (i.e., on MacOS running Chisel tests using Scalatest), this can result in stack overflows. Fix this method to make it recursive. This is similar to https://github.com/llvm/circt/pull/5305.

Start by locating the FIRRTL InferWidths implementation and the checkCycles method, then compare its traversal with the analogous change in LLVM CIRCT pull request 5305. Confirm whether the intended result is iterative, as stated in the title, and verify that cycle checking no longer depends on recursive stack depth.

Written by the indexing model from the issue text.