[ValueTracking] make dominator tree requirement explicit for...

[ValueTracking] make dominator tree requirement explicit for isKnownNonNullFromDominatingCondition(); NFCI

I don't think this hole is currently exposed, but I crashed regression tests for
jump-threading and loop-vectorize after I added calls to isKnownNonNullAt() in
InstSimplify as part of trying to solve PR28430:
https://llvm.org/bugs/show_bug.cgi?id=28430

That's because they call into value tracking with a context instruction, but no
other parts of the query structure filled in.

For more background, see the discussion in:
https://reviews.llvm.org/D27855

llvm-svn: 290786