[coroutines] Promote cleanup.dest.slot allocas to registers to avoid storing...

[coroutines] Promote cleanup.dest.slot allocas to registers to avoid storing it in the coroutine frame

Summary:
We don't want to store cleanup dest slot saved into the coroutine frame (as some of the cleanup code may
access them after coroutine frame destroyed).

This is an alternative to https://reviews.llvm.org/D37093

It is possible to do this for all functions, but, cursory check showed that in -O0, we get slightly longer function (by 1-3 instructions), thus, we are only limiting cleanup.dest.slot elimination to coroutines.

Reviewers: rjmccall, hfinkel, eric_niebler

Reviewed By: eric_niebler

Subscribers: EricWF, cfe-commits

Differential Revision: https://reviews.llvm.org/D39768

llvm-svn: 317981