Revert "Improve prologue handling to support functions with multiple entry points."

This reverts commit r327318. It breaks the Xcode and CMake Darwin
builders:

clang: error: no such file or directory:
'.../source/Plugins/Architecture/PPC64/ArchitecturePPC64.cpp'
clang: error: no input files

More details are in https://reviews.llvm.org/D42582.

llvm-svn: 327327