New exhaustiveness checker breaks concurrent/prog001

The new exhaustiveness checker appears to allocate until the machine runs out of memory while compiling the concurrent/prog001 testcase. In particular it stalls while desugaring the Thread module. The fact that this module contains non-trivial pattern matching, coupled with the compiler gets stuck in Desugar, and the fact that I've noted other performance issues with the new exhaustiveness checker (see #11160 (closed)) leads me to believe that this patch is to blame.