aarch64: fix up regalloc2 semantics. (#4830)

This PR removes all uses of modify-operands in the aarch64 backend,
replacing them with reused-input operands instead. This has the nice
effect of removing a bunch of move instructions and more clearly
representing inputs and outputs.

This PR also removes the explicit use of pinned vregs in the aarch64
backend, instead using fixed-register constraints on the operands when
insts or pseudo-inst sequences require certain registers.

This is the second PR in the regalloc-semantics cleanup series; after
the remaining backend (s390x) and the ABI code are cleaned up as well,
we'll be able to simplify the regalloc2 frontend.

cranelift/codegen/src/isa/aarch64/inst/regs.rs

@@ -50,6 +50,7 @@ pub(crate) const fn vreg_preg(num: u8) -> PReg {
 }
 
 /// Get a writable reference to a V-register.
+#[cfg(test)] // Used only in test code.
 pub fn writable_vreg(num: u8) -> Writable<Reg> {
     Writable::from_reg(vreg(num))
 }