Treat undef bytes as equal to any other byte

compiler/rustc_codegen_llvm/src/common.rs

@@ -126,6 +126,10 @@ impl<'ll, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         unsafe { llvm::LLVMGetUndef(t) }
     }
 
+    fn is_undef(&self, v: &'ll Value) -> bool {
+        unsafe { llvm::LLVMIsUndef(v) == True }
+    }
+
     fn const_poison(&self, t: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMGetPoison(t) }
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -918,6 +918,7 @@ unsafe extern "C" {
     pub fn LLVMMetadataTypeInContext(C: &Context) -> &Type;
 
     // Operations on all values
+    pub fn LLVMIsUndef(Val: &Value) -> Bool;
     pub fn LLVMTypeOf(Val: &Value) -> &Type;
     pub fn LLVMGetValueName2(Val: &Value, Length: *mut size_t) -> *const c_char;
     pub fn LLVMSetValueName2(Val: &Value, Name: *const c_char, NameLen: size_t);