Remove branchTestBool() and simply mask instead to handle bool
return types from C.  The reason for this change is because
otherwise we must hamper the register allocator to guarantee that it
only uses registers with a byte variant (e.g., EAX).  Since the code
in question is the slow path I was reluctant to add uses of
tempFixed() that could result in slower execution paths for the
fast path just to save us one binary AND instruction on the slow
path.