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/x64/lower.isle

@@ -2948,6 +2948,11 @@
 (rule (lower (select_spectre_guard (icmp cc a b) x y))
       (select_icmp (emit_cmp cc a b) x y))
 
+(rule -1 (lower (has_type ty (select_spectre_guard c @ (value_type (fits_in_64 a_ty)) x y)))
+      (let ((size OperandSize (raw_operand_size_of_type a_ty))
+            (gpr_c Gpr (put_in_gpr c)))
+           (with_flags (x64_test size gpr_c gpr_c) (cmove_from_values ty (CC.NZ) x y))))
+
 ;; Rules for `fcvt_from_sint` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule 2 (lower (has_type $F32 (fcvt_from_sint a @ (value_type $I8))))