ABI: implement register arguments with constraints. (#4858)

* ABI: implement register arguments with constraints.

Currently, Cranelift's ABI code emits a sequence of moves from physical
registers into vregs at the top of the function body, one for every
register-carried argument.

For a number of reasons, we want to move to operand constraints instead,
and remove the use of explicitly-named "pinned vregs"; this allows for
better regalloc in theory, as it removes the need to "reverse-engineer"
the sequence of moves.

This PR alters the ABI code so that it generates a single "args"
pseudo-instruction as the first instruction in the function body. This
pseudo-inst defs all register arguments, and constrains them to the
appropriate registers at the def-point. Subsequently the regalloc can
move them wherever it needs to.

Some care was taken not to have this pseudo-inst show up in
post-regalloc disassemblies, but the change did cause a general regalloc
"shift" in many tests, so the precise-output updates are a bit noisy.
Sorry about that!

A subsequent PR will handle the other half of the ABI code, namely, the
callsite case, with a similar preg-to-constraint conversion.

* Update based on review feedback.

* Review feedback.

cranelift/codegen/src/isa/aarch64/inst.isle

@@ -779,6 +779,10 @@
        (CallInd
         (info BoxCallIndInfo))
 
+       ;; A pseudo-instruction that captures register arguments in vregs.
+       (Args
+        (args VecArgPair))
+
        ;; ---- branches (exactly one must appear at end of BB) ----
 
        ;; A machine return instruction.