Ensure that the docs examples verify as Cretonne IL.

Any *.cton files in the docs directory are now included when running the
test-all.sh script. This is to ensure that the examples are in fact
correct IL.

Always print NaN and Inf floats with a sign. Print the positive ones as
+NaN and +Inf to make them easier to parse.

docs/example.cton

@@ -1,18 +1,20 @@
+test verifier
+
 function average(i32, i32) -> f32 {
-    ss1 = stack_slot 8, align 4   ; Stack slot for ``sum``.
+    ss1 = stack_slot 8            ; Stack slot for ``sum``.
 
 ebb1(v1: i32, v2: i32):
     v3 = f64const 0x0.0
     stack_store v3, ss1
     brz v2, ebb3                  ; Handle count == 0.
     v4 = iconst.i32 0
-    br ebb2(v4)
+    jump ebb2(v4)
 
 ebb2(v5: i32):
     v6 = imul_imm v5, 4
     v7 = iadd v1, v6
     v8 = heap_load.f32 v7         ; array[i]
-    v9 = fext.f64 v8
+    v9 = fpromote.f64 v8
     v10 = stack_load.f64 ss1
     v11 = fadd v9, v10
     stack_store v11, ss1
@@ -20,12 +22,12 @@ ebb2(v5: i32):
     v13 = icmp ult v12, v2
     brnz v13, ebb2(v12)           ; Loop backedge.
     v14 = stack_load.f64 ss1
-    v15 = cvt_utof.f64 v2
+    v15 = fcvt_from_uint.f64 v2
     v16 = fdiv v14, v15
-    v17 = ftrunc.f32 v16
+    v17 = fdemote.f32 v16
     return v17
 
 ebb3:
-    v100 = f32const qNaN
+    v100 = f32const +NaN
     return v100
 }