[Analysis] Fix issues when querying vscale attributes on functions

There are several places in the code that are currently broken as
they assume an Instruction always has a parent Function when
attempting to get the vscale_range attribute. This patch adds checks
that an Instruction has a parent.

I've added a test for a parentless @llvm.vscale intrinsic call here:

  unittests/Analysis/ValueTrackingTest.cpp

Differential Revision: https://reviews.llvm.org/D110158