[llvm-profgen] Fix index out of bounds error while using ip.advance

Previously we assume there're some non-executing sections at the bottom of the text section so that we won't hit the array's bound. But on BOLTed binary, it turned out .bolt section is at the bottom of text section which can be profiled, then it crash llvm-profgen. This change try to fix it.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D113238