Fix uniformity issue in CopyTextureForBrowserHelper

Calls to textureSample() must only happen in uniform control flow, so
move them to before the conditional discard.

Bug: tint:880
Change-Id: Idd6ca35a7acaa6917753da8deb6e9266284afe86
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/88463
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/native/CopyTextureForBrowserHelper.cpp

@@ -130,14 +130,15 @@ static const char sCopyTextureForBrowserShader[] = R"(
                 // Clamp the texcoord and discard the out-of-bound pixels.
                 var clampedTexcoord =
                     clamp(texcoord, vec2<f32>(0.0, 0.0), vec2<f32>(1.0, 1.0));
-                if (!all(clampedTexcoord == texcoord)) {
-                    discard;
-                }
 
                 // Swizzling of texture formats when sampling / rendering is handled by the
                 // hardware so we don't need special logic in this shader. This is covered by tests.
                 var color = textureSample(myTexture, mySampler, texcoord);
 
+                if (!all(clampedTexcoord == texcoord)) {
+                    discard;
+                }
+
                 let kUnpremultiplyStep = 0x01u;
                 let kDecodeToLinearStep = 0x02u;
                 let kConvertToDstGamutStep = 0x04u;