Optimize wide decoder case statements into decoder expressions (#7804)

Extend the decoder-pattern case optimization to selectors that are too
wide for a full 2^width lookup table. A decoder-pattern case (where
every case item assigns constants to a fixed set of LHSs) is lowered to
a new AstMachMasked expression. AstMachMasked is emitted as a run-time
VL_MATCHMASKEd_* function call. It contains a packed constant pool table,
'matchp', which is a list of '(mask, bits)' pairs. At runtime, the index of the 
first matching entry is returned, and is used to index a value table. This single
(albeit complicated) expression can replace large if-else trees whole, resulting
in much more compact code with fewer static hard to predict branches. It
is worth about 10% speed and 30% code size in some designs.

Example:

```systemverilog
    logic [39:0] sel;
    always_comb
      casez (sel)
        40'b???????????????????????????????????????1: out = 8'h01;
        40'b??????????????????????????????????????1?: out = 8'h02;
        40'b?????????????????????????????????????1??: out = 8'h03;
        default:                                      out = 8'hff;
      endcase
```

is compiled to:

```c++
    out = TABLE_value[VL_MATCHMASKED_Q(sel, CONST_match)];
```

Where 'CONST_match' contains 4 entries, of a 40-bit mask and 40-bit bit
pattern each, and 'TABLE_value' contains 4 entries of the corresponding
8-bit results. (Entries are aligned to word boundaries to avoid runtime
bit swizzling)

src/V3Gate.cpp

@@ -471,6 +471,11 @@ class GateOkVisitor final : public VNVisitorConst {
         // assign to get randomization etc
         clearSimple("CReset");
     }
+    void visit(AstMatchMasked* nodep) override {
+        if (!m_isSimple) return;
+        // This node can be expensive
+        clearSimple("MatchMasked");
+    }
     //--------------------
     void visit(AstNode* nodep) override {
         if (!m_isSimple) return;  // Fastpath