[CodeGen] Don't require AA in SDAGISel at -O0.

Before r247167, the pass manager builder controlled which AA
implementations were used, exporting them all in the AliasAnalysis
analysis group.

Now, AAResultsWrapperPass always uses BasicAA, but still uses other AA
implementations if made available in the pass pipeline.

But regardless, SDAGISel is required at O0, and really doesn't need to
be doing fancy optimizations based on useful AA results.

Don't require AA at CodeGenOpt::None, and only use it otherwise.

This does have a functional impact (and one testcase is pessimized
because we can't reuse a load).  But I think that's desirable no matter
what.

Note that this alone doesn't result in less DT computations: TwoAddress
was previously able to reuse the DT we computed for SDAG.  That will be
fixed separately.

Differential Revision: https://reviews.llvm.org/D32766

llvm-svn: 302611