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/table.clif

@@ -2,8 +2,8 @@ test verifier
 target x86_64
 
 function %table_base_type(i64 vmctx) {
-    gv0 = vmctx+0
-    gv1 = deref(gv0): i32
+    gv0 = vmctx
+    gv1 = load.i32 notrap aligned gv0
     table0 = dynamic gv1, element_size 1, bound gv1, index_type i32 ; error: table base has type i32, which is not the pointer type i64
 
 ebb0(v0: i64):
@@ -11,7 +11,7 @@ ebb0(v0: i64):
 }
 
 function %invalid_base(i64 vmctx) {
-    gv0 = vmctx+0
+    gv0 = vmctx
     table0 = dynamic gv1, bound gv0, element_size 1, index_type i64 ; error: invalid base global value gv1
 
 ebb0(v0: i64):
@@ -19,7 +19,7 @@ ebb0(v0: i64):
 }
 
 function %invalid_bound(i64 vmctx) {
-    gv0 = vmctx+0
+    gv0 = vmctx
     table0 = dynamic gv0, bound gv1, element_size 1, index_type i64 ; error: invalid bound global value gv1
 
 ebb0(v0: i64):
@@ -27,8 +27,8 @@ ebb0(v0: i64):
 }
 
 function %table_bound_type(i64 vmctx) {
-    gv0 = vmctx+0
-    gv1 = deref(gv0): i16
+    gv0 = vmctx
+    gv1 = load.i16 notrap aligned gv0
     table0 = dynamic gv0, bound gv1, element_size 1, index_type i32 ; error: table index type i32 differs from the type of its bound, i16
 
 ebb0(v0: i64):
@@ -36,8 +36,8 @@ ebb0(v0: i64):
 }
 
 function %table_addr_index_type(i64 vmctx, i64) {
-    gv0 = vmctx+0
-    gv1 = deref(gv0): i32
+    gv0 = vmctx
+    gv1 = load.i32 notrap aligned gv0
     table0 = dynamic gv0, element_size 1, bound gv1, index_type i32
 
 ebb0(v0: i64, v1: i64):