[AArch64] Make instruction fusion more aggressive.

Summary:
This patch makes instruction fusion more aggressive by
* adding artificial edges between the successors of FirstSU and
  SecondSU, similar to BaseMemOpClusterMutation::clusterNeighboringMemOps.
* updating PostGenericScheduler::tryCandidate to keep clusters together,
   similar to GenericScheduler::tryCandidate.

This change increases the number of AES instruction pairs generated on
 Cortex-A57 and Cortex-A72. This doesn't change code at all in
 most benchmarks or general code, but we've seen improvement on kernels
 using AESE/AESMC and AESD/AESIMC. 

Reviewers: evandro, kristof.beyls, t.p.northover, silviu.baranga, atrick, rengolin, MatzeB

Reviewed By: evandro

Subscribers: aemerson, rengolin, MatzeB, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D33230

llvm-svn: 303618