aarch64: Migrate iadd and isub to ISLE

This commit is the first "meaty" instruction added to ISLE for the
AArch64 backend. I chose to pick the first two in the current lowering's
`match` statement, `isub` and `iadd`. These two turned out to be
particularly interesting for a few reasons:

* Both had clearly migratable-to-ISLE behavior along the lines of
  special-casing per type. For example 128-bit and vector arithmetic
  were both easily translateable.

* The `iadd` instruction has special cases for fusing with a
  multiplication to generate `madd` which is expressed pretty easily in
  ISLE.

* Otherwise both instructions had a number of forms where they attempted
  to interpret the RHS as various forms of constants, extends, or
  shifts. There's a bit of a design space of how best to represent this
  in ISLE and what I settled on was to have a special case for each form
  of instruction, and the special cases are somewhat duplicated between
  `iadd` and `isub`. There's custom "extractors" for the special cases
  and instructions that support these special cases will have an
  `rule`-per-case.

Overall I think the ISLE transitioned pretty well. I don't think that
the aarch64 backend is going to follow the x64 backend super closely,
though. For example the x64 backend is having a helper-per-instruction
at the moment but with AArch64 it seems to make more sense to only have
a helper-per-enum-variant-of-`MInst`. This is because the same
instruction (e.g. `ALUOp::Sub32`) can be expressed with multiple
different forms depending on the payload.

It's worth noting that the ISLE looks like it's a good deal larger than
the code actually being removed from lowering as part of this commit. I
think this is deceptive though because a lot of the logic in
`put_input_in_rse_imm12_maybe_negated` and `alu_inst_imm12` is being
inlined into the ISLE definitions for each instruction instead of having
it all packed into the helper functions. Some of the "boilerplate" here
is the addition of various ISLE utilities as well.

cranelift/codegen/src/isa/x64/lower/isle/generated_code.rs

@@ -31,6 +31,7 @@ pub trait Context {
     fn u16_as_u64(&mut self, arg0: u16) -> u64;
     fn u32_as_u64(&mut self, arg0: u32) -> u64;
     fn ty_bits(&mut self, arg0: Type) -> u16;
+    fn fits_in_32(&mut self, arg0: Type) -> Option<Type>;
     fn fits_in_64(&mut self, arg0: Type) -> Option<Type>;
     fn value_list_slice(&mut self, arg0: ValueList) -> ValueSlice;
     fn unwrap_head_value_list_1(&mut self, arg0: ValueList) -> (Value, ValueSlice);