Return iterator from Instruction::eraseFromParent.

Summary:
This is necessary in order to make removal while using reverse iterators work.

(See http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-March/084122.html)

Updates to other eraseFromParent's to come in later patches.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8783

llvm-svn: 233869