Add instructions using CPU flags.

Add integer and floating comparison instructions that return CPU flags:
ifcmp, ifcmp_imm, and ffcmp.

Add conditional branch instructions that check CPU flags: brif, brff

Add instructions that check a condition in the CPU flags and return a
b1: trueif, trueff.

cranelift/filetests/parser/flags.cton

@@ -0,0 +1,46 @@
+test cat
+test verifier
+
+function %iflags(i32) {
+ebb0(v0: i32):
+    v1 = ifcmp_imm v0, 17
+    brif eq v1, ebb1
+    brif ugt v1, ebb2
+    v2 = iconst.i32 34
+    v3 = ifcmp v0, v2
+    v4 = trueif eq v3
+    brnz v4, ebb2
+    return
+
+ebb1:
+    return
+
+ebb2:
+    trap oob
+}
+; check: $v1 = ifcmp_imm $v0, 17
+; check: brif eq $v1, $ebb1
+; check: brif ugt $v1, $ebb2
+; check: $v3 = ifcmp $v0, $v2
+; check: $v4 = trueif eq $v3
+
+function %fflags(f32) {
+ebb0(v0: f32):
+    v1 = f32const 0x34.0p0
+    v2 = ffcmp v0, v1
+    brff eq v2, ebb1
+    brff ord v2, ebb2
+    v3 = trueff gt v2
+    brnz v3, ebb2
+    return
+
+ebb1:
+    return
+
+ebb2:
+    trap oob
+}
+; check: $v2 = ffcmp $v0, $v1
+; check: brff eq $v2, $ebb1
+; check: brff ord $v2, $ebb2
+; check: $v3 = trueff gt $v2