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)
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