[Coroutines] Improve rematerialization stage

As originally implemented, the rematerialization of valid instructions across
the suspend point would iterate 4 times, meaning that up to 4 instructions could
be rematerialized.

This implementation changes that approach to instead build a graph of
rematerializable instructions, then move all of them. This is faster than the
original approach and is not limited to an arbitrary limit.

Differential Revision: https://reviews.llvm.org/D142620