Optimize additional expression patterns in V3Const (#7824)

- Masking that returns known zero
- More general Sel over Extend

src/V3Number.cpp

@@ -1308,6 +1308,14 @@ uint32_t V3Number::mostSetBitP1() const {
     }
     return 0;
 }
+
+uint32_t V3Number::leastSetBitP1() const {
+    for (int bit = 0; bit < width(); ++bit) {
+        if (!bitIs0(bit)) return bit + 1;
+    }
+    return 0;
+}
+
 //======================================================================
 
 V3Number& V3Number::opBitsNonXZ(const V3Number& lhs) {  // 0/1->1, X/Z->0