souper-harvest: Do not generate assignments of constants

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.

cranelift/peepmatic/crates/souper/Cargo.toml

@@ -10,7 +10,7 @@ description = "Converting Souper optimizations into Peepmatic DSL"
 
 [dependencies]
 anyhow = "1"
-souper-ir = { version = "1", features = ["parse"] }
+souper-ir = { version = "2.0.0", features = ["parse"] }
 log = "0.4.8"
 
 [dev-dependencies]