Reorganize the global value kinds. (#490)

* Reorganize the global value kinds.

This:
 - renames "deref" global values to "load" and gives it a offset that works
   like the "load" instructions' does
 - adds an explicit "iadd_imm" global value kind, which replaces the
   builtin iadd in "vmctx" and "deref" global values.
 - also renames "globalsym" to "symbol"

lib/codegen/src/ir/immediates.rs

@@ -214,6 +214,16 @@ impl Offset32 {
     pub fn new(x: i32) -> Self {
         Offset32(x)
     }
+
+    /// Create a new `Offset32` representing the signed numver `x` if possible.
+    pub fn try_from_i64(x: i64) -> Option<Self> {
+        let casted = x as i32;
+        if casted as i64 == x {
+            Some(Self::new(casted))
+        } else {
+            None
+        }
+    }
 }
 
 impl Into<i32> for Offset32 {