De-flake HloProfileTest.ProfileWhileComputation

Instead of guessing that the add instruction is the hottest instruction, scrape
profiles for all instructions and pick the one we want.

Changed the hot instruction for an add to a multiply to avoid ambiguity with the
induction variable increment instruction.

Also changed c_find_if to take a non-const reference, preventing a "auto it =
c_find_if(CreateNewVector(), ...)" dangling pointer bug.  I had to fix up some
(benign looking) code that was calling c_find_if with a temporary to adapt to
this change.

PiperOrigin-RevId: 203849594