Use ConcatIntoIf in const-eval builtin tests.

This CL updates atan2 and atan to use the ConcatIntoIf helper
instead of doing the case concatention.

Bug: tint:1581
Change-Id: I4ea3153aefcab18db88ab81dd499b9f9e37e7906
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/106846
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/tint/resolver/const_eval_builtin_test.cc

@@ -169,8 +169,8 @@ std::vector<Case> Atan2Cases() {
         C({Vec(T(1.0), T(1.0)), Vec(T(0.0), -T(0.0))}, Vec(kPiOver2<T>, kPiOver2<T>)).FloatComp(),
     };
 
-    if constexpr (!finite_only) {
-        std::vector<Case> non_finite_cases = {
+    ConcatIntoIf<!finite_only>(  //
+        cases, std::vector<Case>{
                    // If y is +/-INF and x is finite, +/-PI/2 is returned
                    C({T::Inf(), T(0.0)}, kPiOver2<T>).PosOrNeg().FloatComp(),
                    C({-T::Inf(), T(0.0)}, kPiOver2<T>).PosOrNeg().FloatComp(),
@@ -206,9 +206,7 @@ std::vector<Case> Atan2Cases() {
                      Vec(kPiOver2<T>, kPiOver2<T>, k3PiOver4<T>, k3PiOver4<T>))
                        .PosOrNeg()
                        .FloatComp(),
-        };
-        cases = Concat(cases, non_finite_cases);
-    }
+               });
 
     return cases;
 }
@@ -233,8 +231,8 @@ std::vector<Case> AtanCases() {
         C({Vec(T(0.0), T(1.0), -T(1.0))}, Vec(T(0.0), kPiOver4<T>, -kPiOver4<T>)).FloatComp(),
     };
 
-    if constexpr (!finite_only) {
-        std::vector<Case> non_finite_cases = {
+    ConcatIntoIf<!finite_only>(  //
+        cases, std::vector<Case>{
                    // If i is +/-INF, +/-PI/2 is returned
                    C({T::Inf()}, kPiOver2<T>).PosOrNeg().FloatComp(),
                    C({-T::Inf()}, -kPiOver2<T>).FloatComp(),
@@ -246,9 +244,7 @@ std::vector<Case> AtanCases() {
                    C({Vec(T::Inf(), -T::Inf(), T::Inf(), -T::Inf())},  //
                      Vec(kPiOver2<T>, -kPiOver2<T>, kPiOver2<T>, -kPiOver2<T>))
                        .FloatComp(),
-        };
-        cases = Concat(cases, non_finite_cases);
-    }
+               });
 
     return cases;
 }