brw: Add explicit way to get an empty brw_builder

And use brw_builder(brw_shader *) and brw_builder() constructors
where possible.

The way tests are written, it is necessary to initialize an "empty"
builder -- which is later replaced by a proper one.  Default parameter
NULL make that initialization implicit.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33815>

src/intel/compiler/brw_builder.h

@@ -50,7 +50,14 @@ public:
    {
    }
 
-   explicit brw_builder(brw_shader *s) : brw_builder(s, s->dispatch_width) {}
+   /**
+    * Construct an brw_builder that inserts instructions into \p shader,
+    * using its dispatch width.
+    */
+   explicit brw_builder(brw_shader *s = NULL) :
+      brw_builder(s, s ? s->dispatch_width : 0)
+   {
+   }
 
    /**
     * Construct an brw_builder that inserts instructions before

src/intel/compiler/test_lower_scoreboard.cpp

@@ -42,7 +42,6 @@ protected:
 };
 
 scoreboard_test::scoreboard_test()
-   : bld(NULL, 0)
 {
    ctx = ralloc_context(NULL);
    compiler = rzalloc(ctx, struct brw_compiler);

src/intel/compiler/test_opt_cmod_propagation.cpp

@@ -53,7 +53,6 @@ protected:
 };
 
 cmod_propagation_test::cmod_propagation_test()
-   : bld(NULL, 0)
 {
    ctx = ralloc_context(NULL);
    compiler = rzalloc(ctx, struct brw_compiler);

src/intel/compiler/test_opt_combine_constants.cpp

@@ -68,7 +68,7 @@ struct FSCombineConstantsTest : public ::testing::Test {
 static brw_builder
 make_builder(brw_shader *s)
 {
-   return brw_builder(s, s->dispatch_width).at_end();
+   return brw_builder(s).at_end();
 }
 
 TEST_F(FSCombineConstantsTest, Simple)

src/intel/compiler/test_opt_copy_propagation.cpp

@@ -42,7 +42,6 @@ protected:
 };
 
 copy_propagation_test::copy_propagation_test()
-   : bld(NULL, 0)
 {
    ctx = ralloc_context(NULL);
    compiler = rzalloc(ctx, struct brw_compiler);

src/intel/compiler/test_opt_cse.cpp

@@ -24,7 +24,6 @@ protected:
 };
 
 cse_test::cse_test()
-   : bld(NULL, 0)
 {
    ctx = ralloc_context(NULL);
    compiler = rzalloc(ctx, struct brw_compiler);

src/intel/compiler/test_opt_saturate_propagation.cpp

@@ -42,7 +42,6 @@ protected:
 };
 
 saturate_propagation_test::saturate_propagation_test()
-   : bld(NULL, 0)
 {
    ctx = ralloc_context(NULL);
    compiler = rzalloc(ctx, struct brw_compiler);