[spv-out] Fix invalid spirv being generated from integer dot products (#2291)

* Fix invalid spirv generation in int dotprod

constants cannot be declared inside of a function block, so instead use `write_constant_null` to produce a correctly-declared constant 0.

* autogenerated test snapshots

src/back/spv/block.rs

@@ -1627,12 +1627,7 @@ impl<'w> BlockContext<'w> {
         size: u32,
         block: &mut Block,
     ) {
-        let const_null = self.gen_id();
-        block
-            .body
-            .push(Instruction::constant_null(result_type_id, const_null));
-
-        let mut partial_sum = const_null;
+        let mut partial_sum = self.writer.write_constant_null(result_type_id);
         let last_component = size - 1;
         for index in 0..=last_component {
             // compute the product of the current components

tests/out/spv/functions.spvasm

@@ -23,10 +23,10 @@ OpExecutionMode %74 LocalSize 1 1 1
 %15 = OpTypeVector %7 4
 %18 = OpTypeFunction %12
 %26 = OpTypeFunction %7
-%75 = OpTypeFunction %2
 %31 = OpConstantNull  %7
 %42 = OpConstantNull  %9
 %57 = OpConstantNull  %7
+%75 = OpTypeFunction %2
 %17 = OpFunction  %12  None %18
 %16 = OpLabel
 OpBranch %19

tests/out/spv/math-functions.spvasm

@@ -24,12 +24,12 @@ OpEntryPoint Vertex %18 "main"
 %19 = OpTypeFunction %2
 %27 = OpConstantComposite  %14  %5 %5 %5 %5
 %28 = OpConstantComposite  %14  %3 %3 %3 %3
+%31 = OpConstantNull  %7
 %42 = OpConstant  %9  32
 %50 = OpTypeVector %9 2
 %53 = OpConstantComposite  %50  %42 %42
 %65 = OpConstant  %7  31
 %71 = OpConstantComposite  %15  %65 %65
-%31 = OpConstantNull  %7
 %18 = OpFunction  %2  None %19
 %17 = OpLabel
 OpBranch %20