[Coroutines] Salvege Debug.values

Summary: The previous implementation of coro-split didn't collect values
used by dbg instructions into the spills which made a log debug info
unavailable with optimization on.
This patch tries to collect these uses which are used by dbg.values. In
this way, the debugbility of coroutine could be as powerful as normal
functions with optimization on.

To avoid enlarging the coroutine frame, this patch only collects
`dbg.value` whose value is already in the coroutine frame. This decision
may make some debug info getting unavailable. But if we are with
optimization on, the performance issue should be considered first. And
this patch would make the debugbility of coroutine to be better only
without changing the layout of the frame.

Test-plan: check-llvm

Reviewed By: aprantl, lxfind

Differential Revision: https://reviews.llvm.org/D97673