Define icmp and fcmp comparison instructions.

Add new intcc and floatcc operand types for the immediate condition codes on
these instructions.

Add new IntCompare and FloatCompare instruction formats.

Add a generic match_enum() parser function that can match any identifier-like
enumerated operand kind that implements FromStr.

Define the icmp and fcmp instructions in case.py. Include documentation for the
condition codes with these two instructions.

cranelift/tests/parser/tiny.cton.ref

@@ -21,3 +21,17 @@ ebb0:
     v1 = extractlane v0, 3
     v2 = insertlane v0, 1, v1
 }
+
+function icmp(i32, i32) {
+ebb0(vx0: i32, vx1: i32):
+    v0 = icmp eq, vx0, vx1
+    v1 = icmp ult, vx0, vx1
+    v2 = icmp sge, vx0, vx1
+}
+
+function fcmp(f32, f32) {
+ebb0(vx0: f32, vx1: f32):
+    v0 = fcmp eq, vx0, vx1
+    v1 = fcmp uno, vx0, vx1
+    v2 = fcmp lt, vx0, vx1
+}