[IFUNC] Use GlobalIndirectSymbol when aliases and ifuncs have something similar

Second part extracted from http://reviews.llvm.org/D15525

Use GlobalIndirectSymbol in all cases when aliases and ifuncs have
something in common.

Differential Revision: http://reviews.llvm.org/D18754

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265382 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed