Declare constants in the function preamble

This allows us to give names to constants in the constant pool and then use these names in the function body. The original behavior, specifiying the constant value as an instruction immediate, is still supported as a shortcut but some filetests had to change since the canonical way of printing the CLIF constants is now in the preamble.

cranelift/frontend/src/frontend.rs

@@ -1262,13 +1262,15 @@ block0:
         assert_eq!(
             func.display(None).to_string(),
             "function %sample() -> i8x16, b8x16, f32x4 system_v {
+    const0 = 0x00000000000000000000000000000000
+
 block0:
     v5 = f32const 0.0
     v6 = splat.f32x4 v5
     v2 -> v6
-    v4 = vconst.b8x16 0x00
+    v4 = vconst.b8x16 const0
     v1 -> v4
-    v3 = vconst.i8x16 0x00
+    v3 = vconst.i8x16 const0
     v0 -> v3
     return v0, v1, v2
 }