Make SkASSERT not break build in constexpr GrSwizzle method on GCC

Change-Id: I042b90e7c405505447662e6d187ca1519efd4743
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221342
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

src/gpu/GrSwizzle.h

@@ -117,7 +117,9 @@ constexpr float GrSwizzle::ComponentIndexToFloat(const SkPMColor4f& color, int i
     if (idx == CToI('0')) {
         return 1.0f;
     }
-    SkASSERT(false);
+    bool failed = true;  // Using this var allows SkASSERT() to be used on GCC.
+    (void)failed;        // Otherwise unused in release build.
+    SkASSERT(!failed);
     return -1.0f;
 }