Add -enable-aa-sched-mi, off by default, for AliasAnalysis inside MachineScheduler.

This feature avoids creating edges in the scheduler's dependence graph
for non-aliasing memory operations according to whichever alias
analysis is available. It has been fully tested in Hexagon. Before
making this default, it needs to be extended to handle multiple
MachineMemOperands, compile time needs more evaluation, and
benchmarking on X86 and ARM is needed.

Patch by Sergei Larin!

llvm-svn: 156842