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/riscv64/lower.isle

@@ -831,6 +831,9 @@
       (_ Unit (emit (MInst.SelectIf $true (vec_writable_clone dst) r a b))))
     (vec_writable_to_regs dst)))
 
+(rule -1
+  (lower (has_type ty (select_spectre_guard c @ (value_type cty) x y)))
+  (gen_select ty (normalize_cmp_value cty c) x y))
 
 ;;;;;  Rules for `bmask`;;;;;;;;;
 (rule