[CaptureTracking] Allow null to be in either icmp operand

[CaptureTracking] Allow null to be in either icmp operand

Summary:
Before we required the comparison against null to be "canonical", hence
null to be operand #1. This patch allows null to be in either operand,
similar to the handling of loaded globals that follows.

Reviewers: sanjoy, hfinkel, aykevl, sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66321

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369158 91177308-0d34-0410-b5e6-96231b3b80d8