[PGO] Fix handling of cold entry count for instrumented PGO

Summary:
In r277849, getEntryCount was changed to return None when the entry
count was 0, specifically for SamplePGO where it means no samples were
recorded. However, for instrumentation PGO a 0 entry count should be
returned directly, since it does mean that the function was completely
cold. Otherwise we end up treating these functions conservatively
in isFunctionEntryCold() and isColdBB().

Instead, for SamplePGO use -1 when there are no samples, and change
getEntryCount to return None when the value is -1.

Reviewers: danielcdh, davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41307

llvm-svn: 321018