[InstCombine] Pass a proper context instruction to all of the calls into InstSimplify

Summary: This matches the behavior we already had for compares and makes us consistent everywhere.

Reviewers: dberlin, hfinkel, spatel

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33604

llvm-svn: 305049