[ValueTracking] avoid crashing from bad assumptions (PR31809)

A program may contain llvm.assume info that disagrees with other analysis. 
This may be caused by UB in the program, so we must not crash because of that.

As noted in the code comments:
https://llvm.org/bugs/show_bug.cgi?id=31809
...we can do better, but this at least avoids the assert/crash in the bug report.

Differential Revision: https://reviews.llvm.org/D29395

llvm-svn: 293773