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/V3Dfg.cpp

@@ -96,6 +96,11 @@ std::unique_ptr<DfgGraph> DfgGraph::clone() const {
             vtxp2clonep.emplace(&vtx, cp);
             break;
         }  // LCOV_EXCL_STOP
+        case VDfgType::MatchMasked: {
+            DfgMatchMasked* const cp = new DfgMatchMasked{*clonep, vtx.fileline(), vtx.dtype()};
+            vtxp2clonep.emplace(&vtx, cp);
+            break;
+        }
         case VDfgType::Sel: {
             DfgSel* const cp = new DfgSel{*clonep, vtx.fileline(), vtx.dtype()};
             cp->lsb(vtx.as<DfgSel>()->lsb());
@@ -672,6 +677,15 @@ void DfgVertex::typeCheck(const DfgGraph& dfg) const {
         CHECK(v.dtype() == DfgDataType::select(v.srcp()->dtype(), v.lsb(), v.size()), "sel");
         return;
     }
+    case VDfgType::MatchMasked: {
+        const DfgMatchMasked& v = *as<DfgMatchMasked>();
+        CHECK(v.isPacked(), "Should be Packed type");
+        CHECK(v.size() == 32U, "Should yield a 32-bit result");
+        CHECK(v.lhsp()->isPacked(), "Lhs should be packed");
+        CHECK(v.matchp()->isPacked(), "Match should be Packed type");
+        CHECK(v.matchp()->is<DfgVertexVar>(), "Match should be a variable");
+        return;
+    }
     case VDfgType::Mux: {
         const DfgMux& v = *as<DfgMux>();
         CHECK(v.isPacked(), "Should be Packed type");