[InlineCost, NFC] Extract code dealing with inbounds GEPs from visitGetElementPtr...

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314787 91177308-0d34-0410-b5e6-96231b3b80d8