Allowing singular extension in mate positions

Drop useless condition

abs(ttValue) < VALUE_KNOWN_WIN

And extend singular extension search to cases when ttValue
stores a mate score. This improves mate finding and does
not introduce any regression.

Yery tested this patch against current master on the 6500+
Chest mate suite with 200K fixed nodes:

    shortest mates found: master: 1206 patch:1205
    any mate found: master: 1903 patch: 2003

with 1 sec time:

    shortest mates found: master: 2667 patch: 2628
    any mate found: master: 3585 patch: 3646

Verified for no regression:

STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 25655 W: 4578 L: 4465 D: 16612

LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 66247 W: 8618 L: 8557 D: 49072

bench: 6335042

src/search.cpp

@@ -845,8 +845,7 @@ namespace {
     singularExtensionNode =   !rootNode
                            &&  depth >= 8 * ONE_PLY
                            &&  ttMove != MOVE_NONE
-                       /*  &&  ttValue != VALUE_NONE Already implicit in the next condition */
-                           &&  abs(ttValue) < VALUE_KNOWN_WIN
+                           &&  ttValue != VALUE_NONE
                            && !excludedMove // Recursive singular search is not allowed
                            && (tte->bound() & BOUND_LOWER)
                            &&  tte->depth() >= depth - 3 * ONE_PLY;
@@ -904,7 +903,7 @@ namespace {
           && !extension
           &&  pos.legal(move))
       {
-          Value rBeta = ttValue - 2 * depth / ONE_PLY;
+          Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
           Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
           ss->excludedMove = move;
           ss->skipEarlyPruning = true;