Cranelift: Add instructions for getting the current stack/frame/return pointers (#4573)

* Cranelift: Add instructions for getting the current stack/frame pointers and return address

This is the initial part of https://github.com/bytecodealliance/wasmtime/issues/4535

* x64: Remove `Amode::RbpOffset` and use `Amode::ImmReg` instead

We just special case getting operands from `Amode`s now.

* Fix s390x `get_return_address`; require `preserve_frame_pointers=true`

* Assert that `Amode::ImmRegRegShift` doesn't use rbp/rsp

* Handle non-allocatable registers in Amode::with_allocs

* Use "stack" instead of "r15" on s390x

* r14 is an allocatable register on s390x, so it shouldn't be used with `MovPReg`

cranelift/codegen/src/isa/s390x/lower/isle.rs

@@ -21,6 +21,7 @@ use crate::{
     isa::unwind::UnwindInst,
     machinst::{InsnOutput, LowerCtx, VCodeConstant, VCodeConstantData},
 };
+use regalloc2::PReg;
 use std::boxed::Box;
 use std::cell::Cell;
 use std::convert::TryFrom;
@@ -603,6 +604,11 @@ where
         MemArg::reg_plus_off(stack_reg(), base + off, MemFlags::trusted())
     }
 
+    #[inline]
+    fn memarg_initial_sp_offset(&mut self, off: i64) -> MemArg {
+        MemArg::InitialSPOffset { off }
+    }
+
     #[inline]
     fn memarg_symbol(&mut self, name: ExternalName, offset: i32, flags: MemFlags) -> MemArg {
         MemArg::Symbol {
@@ -670,6 +676,11 @@ where
     fn emit(&mut self, inst: &MInst) -> Unit {
         self.lower_ctx.emit(inst.clone());
     }
+
+    #[inline]
+    fn preg_stack(&mut self) -> PReg {
+        stack_reg().to_real_reg().unwrap().into()
+    }
 }
 
 /// Zero-extend the low `from_bits` bits of `value` to a full u64.