[x86, AVX] don't add a vzeroupper if that's what the code is already doing (PR27823)

This isn't the complete fix, but it handles the trivial examples of duplicate vzero* ops in PR27823:
https://llvm.org/bugs/show_bug.cgi?id=27823
...and amusingly, the bogus cases already exist as regression tests, so let's take this baby step.

We'll need to do more in the general case where there's legitimate AVX usage in the function + there's
already a vzero in the code.

Differential Revision: http://reviews.llvm.org/D20477

llvm-svn: 270378