fix for SkSL producing invalid branch after an OpKill

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2129913002

Review-Url: https://codereview.chromium.org/2129913002

src/sksl/SkSLSPIRVCodeGenerator.cpp

@@ -775,6 +775,7 @@ void SPIRVCodeGenerator::writeOpCode(SpvOp_ opCode, int length, std::ostream& ou
     switch (opCode) {
         case SpvOpReturn:      // fall through
         case SpvOpReturnValue: // fall through
+        case SpvOpKill:        // fall through
         case SpvOpBranch:      // fall through
         case SpvOpBranchConditional:
             ASSERT(fCurrentBlock);