Cranelift: implement general select_spectre_guard fallbacks. (#5420)

When adding some optimization rules for `icmp` in the egraph
infrastructure, we ended up creating a path to legal CLIF but with
patterns unsupported by three of our four backends: specifically,
`select_spectre_guard` with a general truthy input, rather than an
`icmp`.

In #5206 we discussed replacing `select_spectre_guard` with something
more specific, and that could still be a long-term solution here, but
doing so now would interfere with ongoing refactoring of heap access
lowering, so I've opted not to do so. (In that issue I was concerned
about complexity and didn't see the need but with this fuzzbug I'm
starting to feel a bit differently; maybe we should remove this
non-orthogonal op in the long run.)

Fixes #5417.

cranelift/codegen/src/isa/aarch64/lower.isle

@@ -1746,6 +1746,12 @@
             (_ InstOutput (side_effect (csdb))))
        dst))
 
+(rule -1 (lower (has_type ty (select_spectre_guard rcond rn rm)))
+      (let ((rcond Reg (put_in_reg_zext64 rcond)))
+       (lower_select
+        (cmp (OperandSize.Size64) rcond (zero_reg))
+        (Cond.Ne) ty rn rm)))
+
 ;;;; Rules for `vconst` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule (lower (has_type (ty_vec128 _) (vconst (u128_from_constant x))))