[codegen] legalize icmp for 64 and 128 bit operands

Add legalizations for icmp and icmp_imm for i64 and i128 operands for
the narrow legalization set, allowing 32-bit ISAs (like x86-32) to
compare 64-bit integers and all ISAs to compare 128-bit integers.

Fixes: https://github.com/bnjbvr/cranelift-x86/issues/2

cranelift/filetests/filetests/isa/x86/icmp-i128.clif

@@ -0,0 +1,52 @@
+test run
+target x86_64 haswell
+
+function %test_icmp_eq_i128() -> b1 {
+ebb0:
+    v11 = iconst.i64 0x0
+    v12 = iconst.i64 0x0
+    v1 = iconcat v11, v12
+    v21 = iconst.i64 0x0
+    v22 = iconst.i64 0x0
+    v2 = iconcat v21, v22
+    v10 = icmp.i128 eq v1, v2
+    return v10
+}
+
+; run
+
+function %test_icmp_imm_eq_i128() -> b1 {
+ebb0:
+    v11 = iconst.i64 0x0
+    v12 = iconst.i64 0x0
+    v1 = iconcat v11, v12
+    v10 = icmp_imm.i128 eq v1, 0x0
+    return v10
+}
+
+; run
+
+function %test_icmp_ne_i128() -> b1 {
+ebb0:
+    v11 = iconst.i64 0x0
+    v12 = iconst.i64 0x0
+    v1 = iconcat v11, v12
+    v21 = iconst.i64 0x0
+    v22 = iconst.i64 0x1
+    v2 = iconcat v21, v22
+    v10 = icmp.i128 ne v1, v2
+    return v10
+}
+
+; run
+
+function %test_icmp_imm_ne_i128() -> b1 {
+ebb0:
+    v11 = iconst.i64 0x0
+    v12 = iconst.i64 0x0
+    v1 = iconcat v11, v12
+    v10 = icmp_imm.i128 ne v1, 0x1
+    return v10
+}
+
+; run