Fix a -Wmaybe-uninitialized warning

`MDWrapper::get` may not fill its argument in error cases, so
initialize `Arg` to an all-ones value.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@2c247b7

Author: svenvh

llvm-spirv/lib/SPIRV/SPIRVWriter.cpp

@@ -4493,6 +4493,12 @@ bool LLVMToSPIRVBase::transExecutionMode() {
       if (!BF)
         return false;
 
+      auto AddSingleArgExecutionMode = [&](ExecutionMode EMode) {
+        uint32_t Arg = ~0u;
+        N.get(Arg);
+        BF->addExecutionMode(BM->add(new SPIRVExecutionMode(BF, EMode, Arg)));
+      };
+
       switch (EMode) {
       case spv::ExecutionModeContractionOff:
         BF->addExecutionMode(BM->add(