Issue Description:

We use -solver-memory-threshold as a way to keep compile times low in our large codebase in a consistent manner (as opposed to -warn-long-function-bodies which varies from machine to machine).

With Swift 4.1, we notice the warnings no longer seem to appear. I suspect it's due to the changes introduced in 5f2aeec.

My guess is that this clause:

// If we haven't explored a relatively large number of possibilities
// yet, continue.
if (CountScopes <= 16 * 1024)
    return false; 

causes an early exit from the getExpressionTooComplex() and it never gets to the end of the fucntion, where the memory threshold check is done.

It would be good to move the memory check early so that it's still honored for those users who do wish to limit the complexity of expressions allowed in their codebase.

If this is not reasonable, then perhaps we could allow user to tune the hard-coded 16K number in the number of scopes check in the code snippet above.