Do not do IIR in singular search nodes (#378)

Passed STC nonreg
```
Test  | no-iir-se
Elo   | -0.17 +- 1.28 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=16MB
LLR   | 2.95 (-2.94, 2.94) [-3.00, 0.00]
Games | N: 98098 W: 25454 L: 25502 D: 47142
Penta | [1273, 11825, 22881, 11817, 1253]
```
https://ob.cwchess.org/test/1263/

Passed LTC nonreg
```
Test  | no-iir-se
Elo   | 0.11 +- 1.37 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=64MB
LLR   | 2.97 (-2.94, 2.94) [-3.00, 0.00]
Games | N: 69620 W: 16839 L: 16816 D: 35965
Penta | [382, 8268, 17509, 8247, 404]
```
https://ob.cwchess.org/test/1266/

Bench: 16617307

Author: kevlu8

src/search.cpp

@@ -475,7 +475,8 @@ Value Worker::search(
     }
 
     // Internal Iterative Reductions
-    if ((PV_NODE || cutnode) && depth >= 8 && (!tt_data || tt_data->move == Move::none())) {
+    if ((PV_NODE || cutnode) && depth >= 8 && !excluded
+        && (!tt_data || tt_data->move == Move::none())) {
         depth--;
     }