Add useAA() to TargetSubtargetInfo

There are several optional (off-by-default) features in CodeGen that can make
use of alias analysis. These features are important for generating code for
some kinds of cores (for example the (in-order) PPC A2 core). This adds a
useAA() function to TargetSubtargetInfo to allow these features to be enabled
by default on a per-subtarget basis.

Here is the first use of this function: To control the default of the
-enable-aa-sched-mi feature.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189563 91177308-0d34-0410-b5e6-96231b3b80d8