[CSSPGO][llvm-profgen] Fix an issue in findDisjointRanges

We were using 0 as an indicator of invalid offset when computing disjoint ranges. In reality, 0 can be an valid code offset which stands for the first function in .text section. I'm using UINT64_MAX as an invalid code offset instead.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D104497