[CodeExtractor] Fix multiple bugs under certain shape of extracted region

Summary:
If the extracted region has multiple exported data flows toward the same BB which is not included in the region, correct resotre instructions and PHI nodes won't be generated inside the exitStub. The solution is simply put the restore instructions right after the definition of output values instead of putting in exitStub.
Unittest for this bug is included.

Author: myhsu

Reviewers: chandlerc, davide, lattner, silvas, davidxl, wmi, kuhar

Subscribers: dberlin, kuhar, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D37902

llvm-svn: 315041