-mlir-timing isn't aware of intra-pass parallelization

The `-mlir-timing` flag does some rejiggering when the pass manager runs passes in parallel, but it isn't aware of internally parallelized passes. For example, the user and wall time of IMConstProp are always the same even though it is internally threaded.

Start by locating the implementation of the -mlir-timing flag and the IMConstProp pass, then trace how timing is collected for pass-manager parallelism versus pass-internal threading. Done means timing reports distinguish user and wall time for internally parallelized passes such as IMConstProp, with regression coverage in the relevant existing timing tests.

Written by the indexing model from the issue text.