[TTI] Evaluate cost of experimental_widenable_condition as zero

[mkazantsev]

Summary

This intrinsic is not supposed to live through lowering, eventually it should turn
into true constant and be optimized away.

[skatkov]

lgtm

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

InstructionCost getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     case Intrinsic::coro_free:
     case Intrinsic::coro_end:
     case Intrinsic::coro_frame:
     case Intrinsic::coro_size:
     case Intrinsic::coro_align:
     case Intrinsic::coro_suspend:
     case Intrinsic::coro_subfn_addr:
     case Intrinsic::threadlocal_address:
+    case Intrinsic::experimental_widenable_condition:
       // These intrinsics don't actually represent code after lowering.
       return 0;
     }
     return 1;
   }
 
   InstructionCost getCallInstrCost(Function *F, Type *RetTy,
                                    ArrayRef<Type *> Tys,

llvm/test/Transforms/LoopUnroll/guard-cost-for-unrolling.ll

loop:
   br i1 %loop_cond, label %loop, label %exit
 
 exit:
   ret void
 }
 
 define void @test_guard_as_branch(ptr %arr, i64 %n, i64 %bound) {
 ; CHECK-LABEL: Loop Unroll: F[test_guard_as_branch] Loop %loop
-; CHECK-NEXT:    Loop Size = 10
-; CHECK-NEXT:    runtime unrolling with count: 1
+; CHECK-NEXT:    Loop Size = 9
+; CHECK-NEXT:    runtime unrolling with count: 2
 entry:
   br label %loop
 
 loop:                                             ; preds = %guarded, %entry
   %iv = phi i64 [ 0, %entry ], [ %iv.next, %guarded ]
   %gep = getelementptr i64, ptr %arr, i64 %iv
   %bound_check = icmp ult i64 %iv, %bound
   %widenable_cond = call i1 @llvm.experimental.widenable.condition()