Check for empty YMM use-def lists in X86VZeroUpper.

Check for empty YMM use-def lists in X86VZeroUpper.

The previous MRI.isPhysRegUsed(YMM0) would also return true when the
function contains a call to a function that may clobber YMM0. That's
most of them.

Checking the use-def chains allows us to skip functions that don't
explicitly mention YMM registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166110 91177308-0d34-0410-b5e6-96231b3b80d8