intel/fs: Fix flag_subreg handling in cmod propagation

intel/fs: Fix flag_subreg handling in cmod propagation

There were two errors.  First, the pass could propagate conditional
modifiers from an instruction that writes on flag register to an
instruction that writes a different flag register.  For example,

    cmp.nz.f0.0(16) null:F, vgrf6:F, vgrf5:F
    cmp.nz.f0.1(16) null:F, vgrf6:F, vgrf5:F

could be come

    cmp.nz.f0.0(16) null:F, vgrf6:F, vgrf5:F

Second, if an instruction writes f0.1 has it's condition propagated, the
modified instruction will incorrectly write flag f0.0.  For example,

    linterp(16) vgrf6:F, g2:F, attr0:F
    cmp.z.f0.1(16) null:F, vgrf6:F, vgrf5:F
    (-f0.1) discard_jump(16) (null):UD

could become

    linterp.z.f0.0(16) vgrf6:F, g2:F, attr0:F
    (-f0.1) discard_jump(16) (null):UD

None of these cases will occur currently.  The only time we use f0.1 is
for generating discard intrinsics.  In all those cases, we generate a
squence like:

    cmp.nz.f0.0(16) vgrf7:F, vgrf6:F, vgrf5:F
    (+f0.1) cmp.z(16) null:D, vgrf7:D, 0d
    (-f0.1) discard_jump(16) (null):UD

Due to the mixed types and incompatible conditions, this sequence would
never see any cmod propagation.  The next patch will change this.

No shader-db changes on any Intel platform.

v2: Fix typo in comment in test case subtract_delete_compare_other_flag.
Noticed by Caio.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>