[x86] Fix PR34377 by disabling cmov conversion when we relied on it

[x86] Fix PR34377 by disabling cmov conversion when we relied on it
performing a zext of a register.

On the PR there is discussion of how to more effectively handle this,
but this patch prevents us from miscompiling code.

Differential Revision: https://reviews.llvm.org/D37504

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312620 91177308-0d34-0410-b5e6-96231b3b80d8