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/filetests/filetests/egraph/issue-5417.clif

@@ -0,0 +1,15 @@
+test compile
+set opt_level=speed
+set use_egraphs=true
+target x86_64
+target aarch64
+target s390x
+target riscv64
+
+function %my_fn(i16) system_v {
+block0(v0: i16):
+    v1 = icmp eq v0, v0
+    v2 = select_spectre_guard v1, v1, v1
+    return
+}
+; run: %my_fn(6330)