[PGO] Fix PGO use ICE when there are unreachable BBs

For -O0 there might be unreachable BBs, which breaks the assumption that all the
BBs have an auxiliary data structure.  In this patch, we add another interface
called findBBInfo() so that a nullptr can be returned for the unreachable BBs
(and the callers can ignore those BBs).

This fixes the bug reported
https://llvm.org/bugs/show_bug.cgi?id=31209

Differential Revision: https://reviews.llvm.org/D27280

llvm-svn: 288528