[CodeGen] Do not construct complete LValue base info in trivial cases

Besides obvious code simplification, avoiding explicit creation
of LValueBaseInfo objects makes it easier to make TBAA
information to be part of such objects.

This is part of D38126 reworked to be a separate patch to
simplify review.

Differential Revision: https://reviews.llvm.org/D38695

llvm-svn: 315289