Avoid conversion after only reducing goal.
@janno mentioned this tweak to me yesterday.
Apparently he already made a branch for it, and @jtassaro tested it on LambdaRust. It appears to have a -2.65% performance impact https://coq-speed.mpi-sws.org/d/1QE_dqjiz/coq-compare?orgId=1&var-project=lambda-rust&var-branch1=master&var-commit1=01de0893e38924180b91674844d78f3392802ea2&var-config1=coq-8.9.0&var-branch2=ci%2Fjanno%2Freduction_no_check&var-commit2=de7b70f344a0e4da7b6bca009d43a042bb4b875d&var-config2=coq-8.9.0&var-group=(.*)%2F%5B%5E%2F%5D*&var-metric=instructions so let's merge this!