Add fixed_nonallocatable constraints when appropriate (#5253)

Plumb the set of allocatable registers through the OperandCollector and use it validate uses of fixed-nonallocatable registers, like %rsp on x86_64.

cranelift/codegen/src/isa/mod.rs

@@ -227,6 +227,9 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     /// Get the ISA-independent flags that were used to make this trait object.
     fn flags(&self) -> &settings::Flags;
 
+    /// Get the ISA-dependent MachineEnv for managing register allocation.
+    fn machine_env(&self) -> &regalloc2::MachineEnv;
+
     /// Get the ISA-dependent flag values that were used to make this trait object.
     fn isa_flags(&self) -> Vec<settings::Value>;