spirv: Run repair_ssa if there are discard instructions

SPIR-V's OpKill is a control-flow instruction but NIR's discard is not.
Therefore, it can be valid SPIR-V to have

    if (...) {
        foo = /* something */
    } else {
        discard;
    }
    use(foo);

without any phi between the definition of foo and its use.  This is not
true in NIR, however, because NIR's discard isn't considered
control-flow.  Arguably, this is a NIR bug but making discard control-
flow is a very deep change that can have serious ans subtle
side-effects.   The easier thing to do is just fix up the SSA in case we
have an OpKill which might have gotten us into the above case.

Fixes dEQP-VK.graphicsfuzz.vectors-and-discard-in-function with the new
NIR dominance validation pass enabled.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5288>

src/compiler/spirv/vtn_cfg.c

@@ -686,6 +686,7 @@ vtn_process_block(struct vtn_builder *b,
       return NULL;
 
    case SpvOpKill:
+      b->has_kill = true;
       block->branch_type = vtn_branch_type_discard;
       return NULL;
 
@@ -1367,7 +1368,7 @@ vtn_function_emit(struct vtn_builder *b, struct vtn_function *func,
     * but instructions in the continue may use SSA defs in the loop body.
     * Therefore, we need to repair SSA to insert the needed phi nodes.
     */
-   if (b->has_loop_continue)
+   if (b->has_loop_continue || b->has_kill)
       nir_repair_ssa_impl(func->impl);
 
    func->emitted = true;

src/compiler/spirv/vtn_private.h

@@ -683,6 +683,7 @@ struct vtn_builder {
    unsigned func_param_idx;
 
    bool has_loop_continue;
+   bool has_kill;
 
    /* false by default, set to true by the ContractionOff execution mode */
    bool exact;