[PGO] Fix insane counts due to nonreturn calls

Summary:
Since we don't break BBs for function calls. We might get some insane counts 
(wrap of unsigned) in the presence of noreturn calls.

This patch sets these counts to zero instead of the wrapped number.

Reviewers: davidxl

Subscribers: xur, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D27602

llvm-svn: 289521