Speed up valid hole-fits by adding early abort and checks.

By adding an early abort flag in `TcSEnv`, we can fail fast in the presence of
insoluble constraints. This helps us avoid a lot of work in valid hole-fits, and
we geta massive speed-up by avoiding a lot of useless work solving constraints that
never come into play.

Additionally, we add a simple check for degenerate hole types, such as
when the type of the hole is an immutable type variable (as is the case
when the hole is completely unconstrained). Then the only valid fits are
the locals, so we can ignore the global candidates.

This fixes #16875