[InlineCost, NFC] Change CallAnalyzer::isGEPFree to use TTI::getUserCost instead of TTI::getGEPCost

Currently CallAnalyzer::isGEPFree uses TTI::getGEPCost to check if GEP is free.
TTI::getGEPCost cannot handle cases when GEPs participate in Def-Use dependencies
(see https://reviews.llvm.org/D31186 for example).
There is TTI::getUserCost which can calculate the cost more accurately by
taking dependencies into account.

Differential Revision: https://reviews.llvm.org/D33685

llvm-svn: 309268