[Analysis] Fix another issue when querying vscale attributes on functions

There are several places in the code that are currently broken where
we assume an Instruction is always a member of a BasicBlock that
lives in a Function. This is a problem specifically when
attempting to get the vscale_range attribute. This patch adds checks
that an Instruction's parent also has a parent!

I've added a test for a function-less @llvm.vscale intrinsic call here:

  unittests/Analysis/ValueTrackingTest.cpp