[Coroutines] Allow FramePtr to be an Argument

With opaque pointers, after splitRetconCoroutine() the FramePtr
may be an Argument rather than an Instruction. With typed pointers,
this currently doesn't happen because the FramePtr would be a
bitcast instruction.

Fix this by making FramePtr a Value and adding a helper for the
"after FramePtr" insertion point, which would be the start of the
function in the Argument case.

Differential Revision: https://reviews.llvm.org/D120994