Add a verifier pass for CPU flags.

Only one CPU flags value can be live at a time, and some instructions
clobber the flags.

cranelift/filetests/verifier/flags.cton

@@ -0,0 +1,76 @@
+test verifier
+isa intel
+
+; Simple, correct use of CPU flags.
+function %simple(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op2seti_abcd#490]  v2 = trueif ugt v1
+    [Op2urm_abcd#4b6]   v3 = bint.i32 v2
+    [Op1ret#c3]         return v3
+}
+
+; Overlapping flag values of different types.
+function %overlap(i32, f32) -> i32 {
+                    ebb0(v0: i32, v1: f32):
+    [Op1rcmp#39]        v2 = ifcmp v0, v0
+    [Op2fcmp#42e]       v3 = ffcmp v1, v1
+    [Op2setf_abcd#490]  v4 = trueff gt v3 ; error: conflicting live CPU flags: v2 and v3
+    [Op2seti_abcd#490]  v5 = trueif ugt v2
+    [Op1rr#21]          v6 = band v4, v5
+    [Op2urm_abcd#4b6]   v7 = bint.i32 v6
+    [Op1ret#c3]         return v7
+}
+
+; CPU flags clobbered by arithmetic.
+function %clobbered(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op1rr#01]          v2 = iadd v0, v0 ; error: encoding clobbers live CPU flags in v1
+    [Op2seti_abcd#490]  v3 = trueif ugt v1
+    [Op2urm_abcd#4b6]   v4 = bint.i32 v3
+    [Op1ret#c3]         return v4
+}
+
+; CPU flags not clobbered by load.
+function %live_across_load(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op1ld#8b]          v2 = load.i32 v0
+    [Op2seti_abcd#490]  v3 = trueif ugt v1
+    [Op2urm_abcd#4b6]   v4 = bint.i32 v3
+    [Op1ret#c3]         return v4
+}
+
+; Correct use of CPU flags across EBB.
+function %live_across_ebb(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op1jmpb#eb]        jump ebb1
+                    ebb1:
+    [Op2seti_abcd#490]  v2 = trueif ugt v1
+    [Op2urm_abcd#4b6]   v3 = bint.i32 v2
+    [Op1ret#c3]         return v3
+}
+
+function %live_across_ebb_backwards(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1jmpb#eb]        jump ebb2
+                    ebb1:
+    [Op2seti_abcd#490]  v2 = trueif ugt v1
+    [Op2urm_abcd#4b6]   v3 = bint.i32 v2
+    [Op1ret#c3]         return v3
+                    ebb2:
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op1jmpb#eb]        jump ebb1
+}
+
+; Flags live into loop.
+function %live_into_loop(i32) -> i32 {
+                    ebb0(v0: i32):
+    [Op1rcmp#39]        v1 = ifcmp v0, v0
+    [Op1jmpb#eb]        jump ebb1
+                    ebb1:
+    [Op2seti_abcd#490]  v2 = trueif ugt v1
+    [Op1jmpb#eb]        jump ebb1
+}