intel/fs: Handle surface opcode sample masks via predication.

The main motivation is to enable HDC surface opcodes on ICL which no
longer allows the sample mask to be provided in a message header, but
this is enabled all the way back to IVB when possible because it
decreases the instruction count of some shaders using HDC messages
significantly, e.g. one of the SynMark2 CSDof compute shaders
decreases instruction count by about 40% due to the removal of header
setup boilerplate which in turn makes a number of send message
payloads more easily CSE-able.  Shader-db results on SKL:

 total instructions in shared programs: 15325319 -> 15314384 (-0.07%)
 instructions in affected programs: 311532 -> 300597 (-3.51%)
 helped: 491
 HURT: 1

Shader-db results on BDW where the optimization needs to be disabled
in some cases due to hardware restrictions:

 total instructions in shared programs: 15604794 -> 15598028 (-0.04%)
 instructions in affected programs: 220863 -> 214097 (-3.06%)
 helped: 351
 HURT: 0

The FPS of SynMark2 CSDof improves by 5.09% ±0.36% (n=10) on my SKL
laptop with this change.  According to Eero this improves performance
of the same test by 9% on BYT and by 7-8% on BXT J4205 and on SKL GT2
desktop.

Reviewed-by: Kenneth Graunke <[email protected]>
Tested-By: Eero Tamminen <[email protected]>

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