Add llvm.type.checked.load intrinsic

Add the intrinsic

declare {i8*, i1} @llvm.type.checked.load(i8* %ptr, i32 %offset, metadata %type)

This is used in the VFE optimization when lowering loading functions
from vtables to LLVM IR. The `metadata` is used to map the function to
all vtables this function could belong to. This ensures that functions
from vtables that might be used somewhere won't get removed.

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -406,6 +406,16 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
         self.call_intrinsic("llvm.type.test", &[bitcast, typeid])
     }
 
+    fn type_checked_load(
+        &mut self,
+        llvtable: &'ll Value,
+        vtable_byte_offset: u64,
+        typeid: &'ll Value,
+    ) -> Self::Value {
+        let vtable_byte_offset = self.const_i32(vtable_byte_offset as i32);
+        self.call_intrinsic("llvm.type.checked.load", &[llvtable, vtable_byte_offset, typeid])
+    }
+
     fn va_start(&mut self, va_list: &'ll Value) -> &'ll Value {
         self.call_intrinsic("llvm.va_start", &[va_list])
     }