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_ssa/src/traits/intrinsic.rs

@@ -22,6 +22,14 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
     fn expect(&mut self, cond: Self::Value, expected: bool) -> Self::Value;
     /// Trait method used to test whether a given pointer is associated with a type identifier.
     fn type_test(&mut self, pointer: Self::Value, typeid: Self::Value) -> Self::Value;
+    /// Trait method used to load a function while testing if it is associated with a type
+    /// identifier.
+    fn type_checked_load(
+        &mut self,
+        llvtable: Self::Value,
+        vtable_byte_offset: u64,
+        typeid: Self::Value,
+    ) -> Self::Value;
     /// Trait method used to inject `va_start` on the "spoofed" `VaListImpl` in
     /// Rust defined C-variadic functions.
     fn va_start(&mut self, val: Self::Value) -> Self::Value;