[AArch64] Enable FeatureFuseAES for the generic processor model.

[AArch64] Enable FeatureFuseAES for the generic processor model.

Summary:
Scheduling AESE/AESMC and AESD/AESIMC instruction pairs back-to-back
gives a double digit speedup on benchmarks using those instructions on
Cortex-A processors. In GCC, this optimization is part of the generic
processor model as well.

This change should not have a major performance impact on processors
that do not optimize AES instruction pairs, although I only had access
to Cortex-A processors for benchmarking.

Reviewers: rengolin, kristof.beyls, javed.absar, evandro, silviu.baranga, MatzeB, mcrosier, joelkevinjones, joel_k_jones, bmakam, t.p.northover

Reviewed By: evandro

Subscribers: sbaranga, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D33836

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305457 91177308-0d34-0410-b5e6-96231b3b80d8