Avoid dynamic_cast() for threads where possible.

It's reasonable to trust currentThread and threadReadyQueue, and these eat
a few percentage points of time.
Shave a tiny bit of time out of CallSyscall().
Organize mem access a bit in SasInstance::Mix().

It's a bit faster but this isn't really the hottest part...
Merge pull request #1005 from unknownbrackets/perf

Small perf improvements from iOS testing