[perf] Fast path for coercions

[compiler-errors]

Check if the goal holds before trying to walk through the goal. If it certainly holds (or certainly does not), then don't select anything.

[compiler-errors]

@bors2 try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

⌛ Trying commit 41aaf68 with merge 840dfe0…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 840dfe0 (840dfe0e6a8015d92e12916e9eaa11641c1909cf, parent: d1d8e386c5e84c4ba857f56c3291f73c27e2d62a)

[rust-timer]

Queued 840dfe0 with parent d1d8e38, future comparison URL.
There are currently 4 preceding artifacts in the queue.
It will probably take at least ~6.5 hours until the benchmark run finishes.