[X86] New and improved VZeroUpperInserter optimization.

- Adds support for inserting vzerouppers before tail-calls.
  This is enabled implicitly by having MachineInstr::copyImplicitOps preserve
  regmask operands, which allows VZeroUpperInserter to see where tail-calls use
  vector registers.

- Fixes a bug that caused the previous version of this optimization to miss some
  vzeroupper insertion points in loops. (Loops-with-vector-code that followed
  loops-without-vector-code were mistakenly overlooked by the previous version).

- New algorithm never revisits instructions.

Fixes <rdar://problem/16228798>