CodeGenPrep: add separate hook say when GEPs should be used for sinking. NFCI.

CodeGenPrep: add separate hook say when GEPs should be used for sinking. NFCI.

Up to now, we've decided whether to sink address calculations using GEPs or
normal arithmetic based on the useAA hook, but there are other reasons GEPs
might be preferred. So this patch splits the two questions, with a default
implementation falling back to useAA.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371721 91177308-0d34-0410-b5e6-96231b3b80d8