Fix optimization rules for narrow types: wrap i8 results to 8 bits. (#5409)

* Fix optimization rules for narrow types: wrap i8 results to 8 bits.

This fixes #5405.

In the egraph mid-end's optimization rules, we were rewriting e.g. imuls
of two iconsts to an iconst of the result, but without masking off the
high bits (beyond the result type's width). This was producing iconsts
with set high bits beyond their types' width, which is not legal.

In addition, this PR adds some optimizations to the algebraic rules to
recognize e.g. `x == x` (and all other integer comparison operators) and
resolve to 1 or 0 as appropriate.

* Review feedback.

* Review feedback, again.

cranelift/filetests/filetests/egraph/cprop.clif

@@ -0,0 +1,43 @@
+test optimize
+set opt_level=none
+set use_egraphs=true
+target x86_64
+
+function %f0() -> i8 {
+block0:
+    v1 = iconst.i8 51
+    v2 = imul.i8 v1, v1
+    return v2
+}
+
+; check: v9 = iconst.i8 41
+; nextln: return v9
+
+function %f1() -> i16 {
+block0:
+    v1 = iconst.i16 1
+    v2 = bnot.i16 v1
+    return v2
+}
+
+; check: v3 = iconst.i16 0xfffe
+; nextln: return v3
+
+function %f2(i8) -> i8 {
+block0(v1: i8):
+    v2 = icmp eq v1, v1
+    return v2
+}
+
+; check: v3 = iconst.i8 1
+; check: return v3
+
+function %f3(i8) -> i8 {
+block0(v1: i8):
+    v2 = icmp ne v1, v1
+    return v2
+}
+
+; check: v3 = iconst.i8 0
+; check: return v3
+

cranelift/filetests/filetests/egraph/issue-5405.clif

@@ -0,0 +1,16 @@
+test interpret
+test run
+set opt_level=speed_and_size
+set use_egraphs=true
+target aarch64
+
+function %a(i64) -> i8 system_v {
+block0(v0: i64):
+    v6 = iconst.i8 51
+    v17 = imul v6, v6  ; v6 = 51, v6 = 51
+    v18 = icmp eq v17, v17
+    v52 = imul v18, v18
+    return v52
+}
+
+; run: %a(129) == 1