[Statepoints] Mark gc.result and gc.relocate as readonly

[igor-laevsky]

Summary

Semantically gc.result and gc.relocate can be considered as readonly functions. This will allow llvm more freedom is finding optimal placement for this intrinsics. The only thing we want to prevent is reordering of the gc.relocates across different statepoints. And this shouldn't happen since we model statepoints as writing calls.

[reames]

LGTM (now that the lowering has been fixed, thanks!)

llvm/trunk/include/llvm/IR/Intrinsics.td

 // These are documented in docs/Statepoint.rst
 
 def int_experimental_gc_statepoint : Intrinsic<[llvm_i32_ty],
                                [llvm_i64_ty, llvm_i32_ty,
                                 llvm_anyptr_ty, llvm_i32_ty,
                                 llvm_i32_ty, llvm_vararg_ty],
                                 [Throws]>;
 
-def int_experimental_gc_result   : Intrinsic<[llvm_any_ty], [llvm_i32_ty]>;
+def int_experimental_gc_result   : Intrinsic<[llvm_any_ty], [llvm_i32_ty],
+                                             [IntrReadMem]>;
 def int_experimental_gc_relocate : Intrinsic<[llvm_anyptr_ty],
-                                [llvm_i32_ty, llvm_i32_ty, llvm_i32_ty]>;
+                                [llvm_i32_ty, llvm_i32_ty, llvm_i32_ty],
+                                [IntrReadMem]>;
 
 // Deprecated: will be removed in a couple of weeks
 def int_experimental_gc_result_int : Intrinsic<[llvm_anyint_ty], [llvm_i32_ty]>;
 def int_experimental_gc_result_float : Intrinsic<[llvm_anyfloat_ty],
                                                  [llvm_i32_ty]>;
 def int_experimental_gc_result_ptr : Intrinsic<[llvm_anyptr_ty], [llvm_i32_ty]>;
 
 //===-------------------------- Other Intrinsics --------------------------===//