x64 backend: fix to_amode with constant address (no registers). (#4239)

If an address expression is given to `to_amode` that is completely
constant (no registers at all), then it will produce an `Amode` that has
the resulting constant as an offset, and `(invalid_reg)` as the base.
This is a side-effect of the way we build up the amode step-by-step --
we're waiting to see a register and plug it into the base field. If we
never get a reg though, we need to generate a constant zero into a
register and use that as the base. This PR adds a `finalize_amode`
helper to do just that.

Fixes #4234.

cranelift/filetests/filetests/isa/x64/atomic_cas_const_addr.clif

@@ -0,0 +1,23 @@
+;; See https://github.com/bytecodealliance/wasmtime/issues/4234.
+;;
+;; We just want to verify that this can be compiled successfully.
+
+test compile
+target x86_64
+
+function u0:31() -> i32, i32 system_v {
+                                block0:
+                                    v0 = iconst.i64 0
+                                    v1 = iconst.i32 0
+                                    v2 = iconst.i32 0
+@0004                               v28 = bconst.b1 false
+@0005                               brnz v28, block25
+                                    jump block1
+
+                                block1:
+@0005                               trap unreachable
+
+                                block25:
+@0035                               v85 = atomic_cas.i32 v0, v1, v2
+@0036                               trap user0
+}