[Coroutines] Directly remove unnecessary lifetime intrinsics

The insertSpills() code will currently skip lifetime intrinsic users
when replacing the alloca with a frame reference. Rather than
leaving behind the dead lifetime intrinsics working on the old
alloca, directly remove them. This makes sure the alloca can be
dropped as well.

I noticed this as a regression when converting tests to opaque
pointers. Without opaque pointers, this code didn't really do
anything, because there would usually be a bitcast in between.
The lifetimes would get rewritten to the frame pointer. With
opaque pointers, this code now triggers and leaves behind users
of the old allocas.

Differential Revision: https://reviews.llvm.org/D148240