[clang][CodeGenPGO] Don't use an invalid index when region counts disagree

If we're using an old instrprof profile and the user passes we can get
Decls with children decl counts not matching the what the profile was
written against. In a particular case I was debugging we have 24 decls
in the AST and 22 decls in the profile. Avoid crashing in this case.

Differential Revision: https://reviews.llvm.org/D149504