intel/fs: Handle flag read/write aliasing in needs_src_copy

In order to implement the ballot intrinsic, we do a MOV from flag
register to some GRF.  If that GRF is used in a SEL, cmod propagation
helpfully changes it into a MOV from the flag register with a cmod.
This is perfectly valid but when lower_simd_width comes along, it simply
splits into two instructions which both have conditional modifiers.
This is a problem since we're reading the flag register.  This commit
makes us check whether or not flags_written() overlaps with the flag
values that we are reading via the instruction source and, if we have
any interference, will force us to emit a copy of the source.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: mesa-stable@lists.freedesktop.org

src/intel/compiler/brw_fs.cpp

@@ -5013,7 +5013,9 @@ needs_src_copy(const fs_builder &lbld, const fs_inst *inst, unsigned i)
 {
    return !(is_periodic(inst->src[i], lbld.dispatch_width()) ||
             (inst->components_read(i) == 1 &&
-             lbld.dispatch_width() <= inst->exec_size));
+             lbld.dispatch_width() <= inst->exec_size)) ||
+          (inst->flags_written() &
+           flag_mask(inst->src[i], type_sz(inst->src[i].type)));
 }
 
 /**