i965/fs: Optimize OR with 0 into a MOV

fs_visitor::set_gs_stream_control_data_bits generates some code like
"control_data_bits | stream_id << ((2 * (vertex_count - 1)) % 32)" as
part of EmitVertex.  The first time this (dynamically) occurs in the
shader, control_data_bits is zero.  Many times we can determine this
statically and various optimizations will collaborate to make one of the
OR operands literal zero.

Converting the OR to a MOV usually allows it to be copy-propagated away.
However, this does not happen in at least some shaders (in the assembly
output of shaders/closed/UnrealEngine4/EffectsCaveDemo/301.shader_test,
search for shl).

All of the affected shaders are geometry shaders.

Broadwell and Skylake had similar results. (Skylake shown)
total instructions in shared programs: 14375452 -> 14375413 (<.01%)
instructions in affected programs: 6422 -> 6383 (-0.61%)
helped: 39
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.14% max: 2.56% x̄: 1.91% x̃: 2.56%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -2.26% -1.57%
Instructions are helped.

total cycles in shared programs: 531981179 -> 531980555 (<.01%)
cycles in affected programs: 27493 -> 26869 (-2.27%)
helped: 39
HURT: 0
helped stats (abs) min: 16 max: 16 x̄: 16.00 x̃: 16
helped stats (rel) min: 0.60% max: 7.92% x̄: 5.94% x̃: 7.92%
95% mean confidence interval for cycles value: -16.00 -16.00
95% mean confidence interval for cycles %-change: -6.98% -4.90%
Cycles are helped.

No changes on earlier platforms.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>

src/intel/compiler/brw_fs.cpp

@@ -2448,7 +2448,8 @@ fs_visitor::opt_algebraic()
          }
          break;
       case BRW_OPCODE_OR:
-         if (inst->src[0].equals(inst->src[1])) {
+         if (inst->src[0].equals(inst->src[1]) ||
+             inst->src[1].is_zero()) {
             inst->opcode = BRW_OPCODE_MOV;
             inst->src[1] = reg_undef;
             progress = true;