[InlineCost, NFC] Extract code dealing with inbounds GEPs from visitGetElementPtr into a function

The code responsible for analysis of inbounds GEPs is extracted into a separate
function: CallAnalyzer::canFoldInboundsGEP. With the patch SROA
enabling/disabling code is localized at one place instead of spreading across
the code of CallAnalyzer::visitGetElementPtr.

Differential Revision: https://reviews.llvm.org/D38233

llvm-svn: 314787