PrettifyVerilog: infinite loop + growing memory consumption on self-loop unary op

Point `firtool` at this testcase: https://github.com/llvm/circt/blob/745e0809a12c0760ccb47ccd6e37fc48e4933361/test/Dialect/Comb/canonicalization.mlir , and prettify verilog spins and consumes over 30GB memory (at which point I killed it).

Judging by sampling stack, I think happening in `prettifyUnaryOperator` looping through uses, but more investigation required.

Start by running firtool on test/Dialect/Comb/canonicalization.mlir at the reported revision and observe the prettify-verilog failure. Then inspect prettifyUnaryOperator and its use traversal, using the stack-sampling observation as an investigation lead. Done means the testcase completes without spinning or runaway memory consumption.

Written by the indexing model from the issue text.