ARM64 backend, part 3 / 11: MachInst infrastructure.

This patch adds the MachInst, or Machine Instruction, infrastructure.
This is the machine-independent portion of the new backend design. It
contains the implementation of the "vcode" (virtual-registerized code)
container, the top-level lowering algorithm and compilation pipeline,
and the trait definitions that the machine backends will fill in.

This backend infrastructure is included in the compilation of the
`codegen` crate, but it is not yet tied into the public APIs; that patch
will come last, after all the other pieces are filled in.

This patch contains code written by Julian Seward <jseward@acm.org> and
Benjamin Bouvier <public@benj.me>, originally developed on a side-branch
before rebasing and condensing into this patch series. See the `arm64`
branch at `https://github.com/cfallin/wasmtime` for original development
history.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Benjamin Bouvier <public@benj.me>

cranelift/codegen/src/lib.rs

@@ -71,6 +71,7 @@ pub mod flowgraph;
 pub mod ir;
 pub mod isa;
 pub mod loop_analysis;
+pub mod machinst;
 pub mod print_errors;
 pub mod settings;
 pub mod timing;
@@ -90,6 +91,7 @@ mod iterators;
 mod legalizer;
 mod licm;
 mod nan_canonicalization;
+mod num_uses;
 mod partition_slice;
 mod postopt;
 mod predicates;