[ARM] Add missing return to ARMTTIImpl::isLoweredToCall.

I assume we meant to return the result of the call to
BaseT::isLoweredToCall(F).

This might not be a functional change in practice because it would
still hit the default case in the switch and call
BaseT::isLoweredToCall(F) at the end.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D123333