The old code used isInstructionTriviallyDead() and removed instructions
when walking the path from a resume call to function return to check if
the call is in tail position.
However, since the code was walking forwards it was not able to get past
instructions such as:
%gep = getelementptr inbounds i64, ptr %alloc.var, i32 0
%foo = ptrtoint ptr %gep to i64
This patch instead ignores such instructions as long as their values are
not needed. This enables the code to emit tail calls in more situations.
9d1cb18d19