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"

cranelift/filetests/verifier/memory.clif

@@ -1,15 +1,15 @@
 test verifier
 
-function %deref_cycle() {
-    gv1 = deref(gv2)-32: i32 ; error: deref cycle: [gv1, gv2]
-    gv2 = deref(gv1): i32
+function %cycle() {
+    gv0 = load.i32 notrap aligned gv1 ; error: global value cycle: [gv0, gv1]
+    gv1 = load.i32 notrap aligned gv0-32
 
 ebb1:
     return
 }
 
 function %self_cycle() {
-    gv0 = deref(gv0)-32: i32 ; error: deref cycle: [gv0]
+    gv0 = load.i32 notrap aligned gv0 ; error: global value cycle: [gv0]
 
 ebb1:
     return