[Attributor][FIX] checkForAllInstructions, correctly handle declarations

checkForAllInstructions was not handling declarations correctly.
It should have been returning false when it gets called on a declaration

The patch also fixes a test case for AAFunctionReachability for it to be able
to pass after the changes to the checkForAllinstructions.

Differential Revision: https://reviews.llvm.org/D106625