i965/vec4: Fix swizzles on atomic sources.

i965/vec4: Fix swizzles on atomic sources.

Atomic operation sources are scalar values, but we were failing to
select the .x component of the second operand.  For example,

   atomicCounterCompSwapARB(counter, 5u, 10u)

would generate

   mov(8) vgrf4.x:D, 5D
   mov(8) vgrf5.x:D, 10D

   mov(8) vgrf9.x:UD, vgrf4.xyzw:D
   mov(8) vgrf9.y:UD, vgrf5.xyzw:D

which wrongly selects the .y component of vgrf5, so the actual 10u value
would get dead code eliminated.  The swizzle works for the other source,
but both of them ought to be .xxxx.

Fixes the compare and swap CTS tests in:
KHR-GL45.shader_atomic_counter_ops_tests.ShaderAtomicCounterOpsExchangeTestCase

Cc: "17.2 17.1 17.0 13.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>