[ThinLTO] Fix dead stripping analysis for SamplePGO

Summary:
The fix for dead stripping analysis in the case of SamplePGO indirect
calls to local functions (r313151) introduced the possibility of an
infinite loop.

Make sure we check for the value being already live after we update it
for SamplePGO indirect call handling.

Reviewers: danielcdh

Subscribers: mehdi_amini, inglorion, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D38086

llvm-svn: 313766