radeonsi: postponed KILL isn't postponed anymore, but maintains WQM

This restores performance for the drirc workaround, i.e.
KILL_IF does:
   visible = src0 >= 0;
   kill_flag &= visible; // accumulate kills
   amdgcn_kill(wqm_vote(visible)); // kill fully dead quads only

And all helper pixels are killed at the end of the shader:
   amdgcn_kill(kill_flag);

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/amd/common/ac_llvm_build.c

@@ -1405,6 +1405,13 @@ LLVMValueRef ac_build_cvt_pkrtz_f16(struct ac_llvm_context *ctx,
 				  AC_FUNC_ATTR_LEGACY);
 }
 
+LLVMValueRef ac_build_wqm_vote(struct ac_llvm_context *ctx, LLVMValueRef i1)
+{
+	assert(HAVE_LLVM >= 0x0600);
+	return ac_build_intrinsic(ctx, "llvm.amdgcn.wqm.vote", ctx->i1,
+				  &i1, 1, AC_FUNC_ATTR_READNONE);
+}
+
 void ac_build_kill_if_false(struct ac_llvm_context *ctx, LLVMValueRef i1)
 {
 	if (HAVE_LLVM >= 0x0600) {

src/amd/common/ac_llvm_build.h

@@ -265,6 +265,7 @@ LLVMValueRef ac_build_image_opcode(struct ac_llvm_context *ctx,
 				   struct ac_image_args *a);
 LLVMValueRef ac_build_cvt_pkrtz_f16(struct ac_llvm_context *ctx,
 				    LLVMValueRef args[2]);
+LLVMValueRef ac_build_wqm_vote(struct ac_llvm_context *ctx, LLVMValueRef i1);
 void ac_build_kill_if_false(struct ac_llvm_context *ctx, LLVMValueRef i1);
 LLVMValueRef ac_build_bfe(struct ac_llvm_context *ctx, LLVMValueRef input,
 			  LLVMValueRef offset, LLVMValueRef width,

src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c

@@ -69,6 +69,12 @@ static void kil_emit(const struct lp_build_tgsi_action *action,
 	}
 
 	if (ctx->shader->selector->force_correct_derivs_after_kill) {
+		/* LLVM 6.0 can kill immediately while maintaining WQM. */
+		if (HAVE_LLVM >= 0x0600) {
+			ac_build_kill_if_false(&ctx->ac,
+					       ac_build_wqm_vote(&ctx->ac, visible));
+		}
+
 		LLVMValueRef mask = LLVMBuildLoad(builder, ctx->postponed_kill, "");
 		mask = LLVMBuildAnd(builder, mask, visible, "");
 		LLVMBuildStore(builder, mask, ctx->postponed_kill);