Exceptional branches

I am pushing onto a stack and checking for stack overflow so I would like the
assembler equivalent of branching only in the exceptional case of an
overflow. However, LLVM only seems to admit equivalent branches so I am not
sure how to convey this information to LLVM's optimizer. How do I tell LLVM
that one branch of my "if" expression is by far the most likely to occur so
that it runs those blocks together and removes the branch in the common case?