Cranelift AArch64: Fix the get_return_address lowering (#4851)

The previous implementation assumed that nothing had clobbered the
LR register since the current function had started executing, so
it would be incorrect for a non-leaf function, for example, that
contains the `get_return_address` operation right after a call.
The operation is valid only if the `preserve_frame_pointers` flag
is enabled, which implies that the presence of a frame record on
the stack is guaranteed.

Copyright (c) 2022, Arm Limited.

cranelift/codegen/src/verifier/mod.rs

@@ -726,6 +726,8 @@ impl<'a> Verifier<'a> {
                 opcode: Opcode::GetFramePointer | Opcode::GetReturnAddress,
             } => {
                 if let Some(isa) = &self.isa {
+                    // Backends may already rely on this check implicitly, so do
+                    // not relax it without verifying that it is safe to do so.
                     if !isa.flags().preserve_frame_pointers() {
                         return errors.fatal((
                             inst,