It turns out that Souper does not allow a constant to be assigned to a variable,
they may only be used as operands. The 2.0.0 version of the `souper-ir` crate
correctly reflects this. In the `cranelift_codegen::souper_harvest` module, we
need to modify our Souper IR harvester so that it delays converting `iconst` and
`bconst` into Souper IR until their values are used as operands. Finally, some
unit tests in the `peepmatic-souper` crate need some small updates as well.
Conversion from Souper into Peepmatic is implemented with a straightforward,
top-down recursive traversal of the optimization's left- and right-hand side
expression DAGs. Most Souper instructions have a corresponding Peepmatic
instruction. If we run into an instruction where that isn't the case, we skip
that Souper optimization and move on to the next one.
Note that Souper fully supports DAGs, for example:
```text
%0 = var
%1 = add 1, %0
%2 = add %1, %1 ;; Two edges to `%1` makes this a DAG.
```
On the other hand, Peepmatic only currently supports trees, so shared
subexpressions are duplicated:
```text
(iadd (iadd 1 $x)
(iadd 1 $x)) ;; The shared subexpression is duplicated.
```
This does not affect correctness.
