Instead, when the `rebuild-peephole-optimizers` feature is enabled, rebuild them
the first time they are used. This allows peepmatic to run when Cranelift's
`Opcode` is defined and available, which paves the way forward for:
* merging `peepmatic_runtime::operator::Operator` and Cranelift's `Opcode` (we
are wasting a bunch of cycles converting between the two of them), and
* supporting vcode optimizations in `peepmatic`.
Before this patch, running the x64 new backend would require both
compiling with --features experimental_x64 and running with
`use_new_backend`.
This patches changes this behavior so that the runtime flag is not
needed anymore: using the feature flag will enforce usage of the new
backend everywhere, making using and testing it much simpler:
cargo run --features experimental_x64 ;; other CLI options/flags
This also gives a hint at what the meta language generation would look
like after switching to the new backend.
Compiling only with the x64 codegen flag gives a nice compile time speedup.
This ports all of the identity, no-op, simplification, and canonicalization
related optimizations over from being hand-coded to the `peepmatic` DSL. This
does not handle the branch-to-branch optimizations or most of the
divide-by-constant optimizations.
