[ValueTracking] Allow context-sensitive nullness check for non-pointers

Same as D60846 but with a fix for the problem encountered there which
was a missing context adjustment in the handling of PHI nodes.

The test that caused D60846 to be reverted was added in e15ab8f2.

Reviewers: nikic, nlopes, mkazantsev,spatel, dlrobertson, uabelho, hakzsam

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69571