anyRewritableTyVar now looks in RuntimeReps
Commit message:
anyRewritableTyVar now looks in RuntimeReps
Previously, anyRewritableTyVar looked only at the arg and res
of `arg -> res`, but their RuntimeReps are also subject to
rewriting. Easy to fix.
Test case: typecheck/should_compile/T17024
Fixes #17024.
Edited by Richard Eisenberg