[Comb] Fold concat-with-constant into the arms of a mux that has constant operands

Nobody has claimed this yet.

This IR:

```
hw.module @SevenSegmentDecoder(%in: i4) -> (%out: i7) {
%c1_i4 = hw.constant 1 : i4
%c6_i6 = hw.constant 6 : i6
%c-1_i6 = hw.constant -1 : i6
%false = hw.constant false
%0 = comb.icmp eq %in, %c1_i4 : i4
%1 = comb.mux %0, %c6_i6, %c-1_i6 : i6
%2 = comb.concat %false, %1 : (i1, i6) -> i7
hw.output %2 : i7
}
```

Can be simplified by folding the "concat of constant + mux" into the two arms of the mux. This should be profitable if the mux has at least one constant operand.

Start by locating the Comb folding or canonicalization entry points for comb.concat and comb.mux, then reproduce the supplied IR example. Check how constant operands are represented and how the mux arms can receive the concat operation. Done means the example is simplified by folding the concatenation into the mux arms when at least one mux operand is constant, with coverage for the transformation.

Written by the indexing model from the issue text.