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

Summary:
Same as D60846 and D69571 but with a fix for the problem encountered
after them. Both times it was a missing context adjustment in the
handling of PHI nodes.

The reproducers created from the bugs that caused the old commits to be
reverted are included.

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

Subscribers: hiraditya, bollu, asbirlea, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71181