Improve on r192635, ExeDepsFix for avx, and add a test case.

rdar:15221834 False AVX register dependencies cause 5x slowdown on
flops-5/6 and significant slowdown on several others.

This was blocking the switch to MI-Sched.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192669 91177308-0d34-0410-b5e6-96231b3b80d8