[WIP][IR] Make `llvm.assume` `readnone`

[jdoerfert]

Summary

Note: This is not a full commit. No tests were changed. Used for discussion

The llvm.assume intrinsic was historically not readnone because
control dependences need to be preserved. However, the intrinsic can
nowadays be represented as what it is, potentially UB if the condition
is false (or an operand bundle is violated). No effect but potentially
UB is readnone (in contrast to speculatable). While here we add
nosync and nofree to llvm.assume.

[nikic]

This looks like a very reasonable change to me, but I can't say that I understand the full consequences :)

[efriedma]

I think this approach makes sense.

[Tyker]

This change makes a lot of sense, but i don't understand all the consequences either.

[jdoerfert]

I will (try to) do some testing with this and report back.

llvm/include/llvm/IR/Intrinsics.td

def int_prefetch
     : Intrinsic<[], [ llvm_anyptr_ty, llvm_i32_ty, llvm_i32_ty, llvm_i32_ty ],
                 [IntrInaccessibleMemOrArgMemOnly, IntrWillReturn,
                  ReadOnly<ArgIndex<0>>, NoCapture<ArgIndex<0>>,
                  ImmArg<ArgIndex<1>>, ImmArg<ArgIndex<2>>]>;
 def int_pcmarker      : Intrinsic<[], [llvm_i32_ty]>;
 
 def int_readcyclecounter : Intrinsic<[llvm_i64_ty]>;
 
-// The assume intrinsic is marked as arbitrarily writing so that proper
-// control dependencies will be maintained.
+// The assume intrinsic is *not* marked as speculatable but without memory
+// effects. Proper control dependencies will be maintained because weakening
+// them would potentially introduce UB on paths that did not contain it before.
 def int_assume        : Intrinsic<[], [llvm_i1_ty], [IntrWillReturn,
+                                                     IntrNoMem,
+                                                     IntrNoSync,
+                                                     IntrNoFree,
                                                      NoUndef<ArgIndex<0>>]>;
 
 // Stack Protector Intrinsic - The stackprotector intrinsic writes the stack
 // guard to the correct place on the stack frame.
 def int_stackprotector : Intrinsic<[], [llvm_ptr_ty, llvm_ptrptr_ty], []>;
 def int_stackguard : Intrinsic<[llvm_ptr_ty], [], []>;
 
 // A counter increment for instrumentation based profiling.