[X86] Move -x86-use-vzeroupper command line flag into runOnMachineFunction for...

[X86] Move -x86-use-vzeroupper command line flag into runOnMachineFunction for the pass itself rather than the pass pipeline construction

This pass has no dependencies on other passes so conditionally
including it in the pipeline doens't do much. Just move it the
pass itself to keep it isolated.