[SimplifyCFG] Extend SimplifyResume to handle phi of trivial landing pad.

Summary:
This is a fix of D13718. D13718 was committed but then reverted because of the following bug:
https://llvm.org/bugs/show_bug.cgi?id=25299

This patch fixes the issue shown in the bug.

Reviewers: majnemer, reames

Subscribers: jevinskie, llvm-commits

Differential Revision: http://reviews.llvm.org/D14308

llvm-svn: 257277