Avoid branching before ValueList::push_back

Instead of checking whether a threat index is valid and then writing to
a vector if it isn't, we can instead always write to the buffer of the
vector, and only increase the size of the vector if the index is valid.

This saves some branch mispredictions.

passed STC:
https://tests.stockfishchess.org/tests/view/69ceb1689f7a7e3fdfc9a44b
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 205760 W: 53109 L: 52561 D: 100090
Ptnml(0-2): 603, 22552, 56076, 22992, 657

local speedtest shows
```
Result of 100 runs (cycles)
===========================
base (...ockfish.orig) =     296886  +/- 1645
test (...kfish.df0822) =     298862  +/- 1662
speedup %              =      +0.67  +/- 0.19
                         [95% CI; t-statistic]

p(speedup > 0)   =  1.0000
p(speedup > .5%) =  0.9562

CPU: 16 x Intel(R) Core(TM) Ultra 9 185H
Hyperthreading: on
```

closes https://github.com/official-stockfish/Stockfish/pull/6703

No functional change

src/misc.h

@@ -201,6 +201,12 @@ class ValueList {
         assert(size_ < MaxSize);
         values_[size_++] = value;
     }
+    // pushes back value if value < max
+    void push_back_if_lt(const T& value, const T& max) {
+        assert(size_ < MaxSize);
+        values_[size_] = value;
+        size_ += (value < max);
+    }
     const T* begin() const { return values_; }
     const T* end() const { return values_ + size_; }
     const T& operator[](int index) const { return values_[index]; }