Apply noundef metadata to loads of types that do not permit raw init

This matches the noundef attributes we apply on arguments/return types.

compiler/rustc_codegen_llvm/src/builder.rs

@@ -479,6 +479,10 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
             load: &'ll Value,
             scalar: abi::Scalar,
         ) {
+            if !scalar.is_always_valid(bx) {
+                bx.noundef_metadata(load);
+            }
+
             match scalar.value {
                 abi::Int(..) => {
                     if !scalar.is_always_valid(bx) {
@@ -1215,6 +1219,16 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
         }
     }
 
+    fn noundef_metadata(&mut self, load: &'ll Value) {
+        unsafe {
+            llvm::LLVMSetMetadata(
+                load,
+                llvm::MD_noundef as c_uint,
+                llvm::LLVMMDNodeInContext(self.cx.llcx, ptr::null(), 0),
+            );
+        }
+    }
+
     pub fn minnum(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
         unsafe { llvm::LLVMRustBuildMinNum(self.llbuilder, lhs, rhs) }
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -442,6 +442,7 @@ pub enum MetadataType {
     MD_mem_parallel_loop_access = 10,
     MD_nonnull = 11,
     MD_type = 19,
+    MD_noundef = 29,
 }
 
 /// LLVMRustAsmDialect