Use TargetSubtargetInfo::useAA() in DAGCombine

This uses the TargetSubtargetInfo::useAA() function to control the defaults of
the -combiner-alias-analysis and -combiner-global-alias-analysis options.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189564 91177308-0d34-0410-b5e6-96231b3b80d8