[ThinLTO] Fix dead stripping analysis for SamplePGO
The fix for dead stripping analysis in the case of SamplePGO indirect
calls to local functions (r313151) introduced the possibility of an
Make sure we check for the value being already live after we update it
for SamplePGO indirect call handling.
Subscribers: mehdi_amini, inglorion, llvm-commits, eraman
Differential Revision: https://reviews.llvm.org/D38086