[coroutines] Make CoroSplit pass deterministic

coro-split-after-phi.ll test was flaky due to non-determinism in
the coroutine frame construction that was sorting the spill
vector using a pointer to a def as a part of the key.

The sorting was intended to make sure that spills for the same def
are kept together, however, we populate the vector by processing
defs in order, so the spill entires will end up together anyways.

This change removes spill sorting and restores the determinism
in the test.

llvm-svn: 299809