[X86] Add a dependency breaking xor before any gathers with an undef passthru value.

In the instruction encoding, the passthru register is always
tied to the destination register. The CPU scheduler has to wait
for the last writer of this register to finish executing before
the gather can start. This is true even if the initial mask is
all ones so that the passthru will never be used.

By explicitly zeroing the register we can break the false
dependency. The zero idiom is executed completing by the
register renamer and so is immedately considered ready.

Authored by Craig.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D112505