Tweak nnue_accumulator indexing

```
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 92736 W: 24149 L: 23764 D: 44823
Ptnml(0-2): 280, 10056, 25334, 10395, 303
```

The use of `IndexType` in the loops is suboptimal because it requires
truncation to 32 bits, and thereby defeats some optimizations. Using
`size_t` for the loop body works nicely, and a signed index into the
index lists allows the compiler to assume the case `added.size() ==
UINT_MAX`, which would be an infinite loop, to not happen.

Passed STC
https://tests.stockfishchess.org/tests/live_elo/692fd98ab23dfeae38d01d98

closes https://github.com/official-stockfish/Stockfish/pull/6466

No functional change

src/misc.h

@@ -134,6 +134,7 @@ class ValueList {
 
    public:
     std::size_t size() const { return size_; }
+    int         ssize() const { return int(size_); }
     void        push_back(const T& value) {
         assert(size_ < MaxSize);
         values_[size_++] = value;