i965/fs: Combine pixel center calculation into one inst.

i965/fs: Combine pixel center calculation into one inst.

The X and Y values come interleaved in g1 (.4-.11 inclusive), so we can
calculate them together with a single add(32) instruction on some
platforms like Broadwell and newer or in SIMD8 elsewhere.

Note that I also moved the PIXEL_X/PIXEL_Y virtual opcodes from before
LINTERP to after it. That's because the writes_accumulator_implicitly()
function in backend_instruction tests for <= LINTERP for determining
whether the instruction indeed writes the accumulator implicitly. The
old FS_OPCODE_PIXEL_X/Y emitted ADD instructions, which did, but the new
opcodes just emit MOVs, which don't. It doesn't matter, since we don't
use these opcodes on Gen4/5 anymore, but in the case that we do...

On Broadwell:
total instructions in shared programs: 7192355 -> 7186224 (-0.09%)
instructions in affected programs:     1190700 -> 1184569 (-0.51%)
helped:                                6131

On Haswell:
total instructions in shared programs: 6155979 -> 6152800 (-0.05%)
instructions in affected programs:     652362 -> 649183 (-0.49%)
helped:                                3179

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>