aarch64: Initial work to transition backend to ISLE (#3541)

* aarch64: Initial work to transition backend to ISLE

This commit is what is hoped to be the initial commit towards migrating
the aarch64 backend to ISLE. There's seemingly a lot of changes here but
it's intended to largely be code motion. The current thinking is to
closely follow the x64 backend for how all this is handled and
organized.

Major changes in this PR are:

* The `Inst` enum is now defined in ISLE. This avoids having to define
  it in two places (once in Rust and once in ISLE). I've preserved all
  the comments in the ISLE and otherwise this isn't actually a
  functional change from the Rust perspective, it's still the same enum
  according to Rust.

* Lots of little enums and things were moved to ISLE as well. As with
  `Inst` their definitions didn't change, only where they're defined.
  This will give future ISLE PRs access to all these operations.

* Initial code for lowering `iconst`, `null`, and `bconst` are
  implemented. Ironically none of this is actually used right now
  because constant lowering is handled in `put_input_in_regs` which
  specially handles constants. Nonetheless I wanted to get at least
  something simple working which shows off how to special case various
  things that are specific to AArch64. In a future PR I plan to hook up
  const-lowering in ISLE to this path so even though
  `iconst`-the-clif-instruction is never lowered this should use the
  const lowering defined in ISLE rather than elsewhere in the backend
  (eventually leading to the deletion of the non-ISLE lowering).

* The `IsleContext` skeleton is created and set up for future additions.

* Some code for ISLE that's shared across all backends now lives in
  `isle_prelude_methods!()` and is deduplicated between the AArch64
  backend and the x64 backend.

* Register mapping is tweaked to do the same thing for AArch64 that it
  does for x64. Namely mapping virtual registers is supported instead of
  just virtual to machine registers.

My main goal with this PR was to get AArch64 into a place where new
instructions can be added with relative ease. Additionally I'm hoping to
figure out as part of this change how much to share for ISLE between
AArch64 and x64 (and other backends).

* Don't use priorities with rules

* Update .gitattributes with concise syntax

* Deduplicate some type definitions

* Rebuild ISLE

* Move isa::isle to machinst::isle

.gitattributes

@@ -11,4 +11,4 @@
 
 # Tell GitHub this is generated code, and doesn't need to be shown in diffs by
 # default.
-cranelift/codegen/src/isa/x64/lower/isle/generated_code.rs linguist-generated
+cranelift/codegen/src/isa/*/lower/isle/generated_code.rs linguist-generated