i965/fs: Add FS_OPCODE_MOV_DISPATCH_TO_FLAGS to fragment shader backend.

i965/fs: Add FS_OPCODE_MOV_DISPATCH_TO_FLAGS to fragment shader backend.

In order to compute centroid varyings correctly, the fragment shader
needs to be able to load the current pixel/sample mask into a flag
register.  This patch adds an opcode to the fragment shader back-end
to do this; the opcode gets translated into the instruction

mov(1)  f0<1>UW  g1.14<0,1,0>UW  { align1 WE_all }

Since this instruction clobbers f0, instruction scheduling has to
treat it the same as instructions that have a conditional modifier.

Reviewed-by: Eric Anholt <eric@anholt.net>