Add x86 encoding for SIMD icmp eq

Also adds a predicate for matching the `eq` IntCC code (TODO this should be replaced by something more general)

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

@@ -0,0 +1,24 @@
+test run
+set enable_simd
+
+function %run_icmp_i8x16() -> b8 {
+ebb0:
+    v0 = vconst.i8x16 0x00
+    v1 = vconst.i8x16 0x00
+    v2 = icmp eq v0, v1
+    v3 = extractlane v2, 0
+    return v3
+}
+
+; run
+
+function %run_icmp_i64x2() -> b64 {
+ebb0:
+    v0 = vconst.i64x2 0xffffffffffffffffffffffffffffffff
+    v1 = vconst.i64x2 0xffffffffffffffffffffffffffffffff
+    v2 = icmp eq v0, v1
+    v3 = extractlane v2, 1
+    return v3
+}
+
+; run