[InstSimplify] Add basic fold for @llvm.cheri.cap.flags.set

If we are setting the same value we just read we can simplify the
sequence to just the original argument.

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -6061,6 +6061,12 @@ static Value *simplifyBinaryIntrinsic(Function *F, Value *Op0, Value *Op1,
         return Op0;
     }
     break;
+  case Intrinsic::cheri_cap_flags_set:
+    // flags_set(x, flags_get(x)) -> x
+    if (match(Op1,
+              m_Intrinsic<Intrinsic::cheri_cap_flags_get>(m_Specific(Op0))))
+      return Op0;
+    break;
   case Intrinsic::cheri_cap_offset_set:
   case Intrinsic::cheri_cap_address_set: {
     Value *Base = getBasePtrIgnoringCapabilityAddressManipulation(Op0, Q.DL);

llvm/test/Transforms/InstSimplify/cheri-intrinsics-get-set.ll

@@ -12,16 +12,16 @@ declare i8 addrspace(200)* @llvm.cheri.cap.INTRINSIC.set.i64(i8 addrspace(200)*,
 
 
 ;; This is a no-op and should be folded to ret %arg
+;; NB: The exception here is @llvm.cheri.cap.high.set since it is tag-clearing
+;; and therefore could only be used on known-untagged values.
 define i8 addrspace(200)* @fold_set_of_get(i8 addrspace(200)* %arg) nounwind {
 ; ADDRESS-LABEL: define {{[^@]+}}@fold_set_of_get
 ; ADDRESS-SAME: (i8 addrspace(200)* [[ARG:%.*]]) addrspace(200) #[[ATTR0:[0-9]+]] {
 ; ADDRESS-NEXT:    ret i8 addrspace(200)* [[ARG]]
 ;
 ; FLAGS-LABEL: define {{[^@]+}}@fold_set_of_get
 ; FLAGS-SAME: (i8 addrspace(200)* [[ARG:%.*]]) addrspace(200) #[[ATTR0:[0-9]+]] {
-; FLAGS-NEXT:    [[VALUE:%.*]] = tail call i64 @llvm.cheri.cap.flags.get.i64(i8 addrspace(200)* [[ARG]])
-; FLAGS-NEXT:    [[RET:%.*]] = tail call i8 addrspace(200)* @llvm.cheri.cap.flags.set.i64(i8 addrspace(200)* [[ARG]], i64 [[VALUE]])
-; FLAGS-NEXT:    ret i8 addrspace(200)* [[RET]]
+; FLAGS-NEXT:    ret i8 addrspace(200)* [[ARG]]
 ;
 ; HIGH-LABEL: define {{[^@]+}}@fold_set_of_get
 ; HIGH-SAME: (i8 addrspace(200)* [[ARG:%.*]]) addrspace(200) #[[ATTR0:[0-9]+]] {