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/regalloc/coalescing-207.clif

@@ -5,7 +5,8 @@ target x86_64 haswell
 ;
 ; The coalescer creates a virtual register with two interfering values.
 function %pr207(i64 vmctx, i32, i32) -> i32 system_v {
-    gv0 = vmctx-8
+    gv1 = vmctx
+    gv0 = iadd_imm.i64 gv1, -8
     heap0 = static gv0, min 0, bound 0x5000, guard 0x0040_0000
     sig0 = (i64 vmctx, i32, i32) -> i32 system_v
     sig1 = (i64 vmctx, i32, i32, i32) -> i32 system_v

cranelift/filetests/regalloc/iterate.clif

@@ -104,7 +104,8 @@ ebb1(v31: i64):
 }
 
 function u0:26(i64 vmctx [%r14]) -> i64 [%rax] baldrdash {
-    gv0 = vmctx+48
+    gv1 = vmctx
+    gv0 = iadd_imm.i64 gv1, 48
     sig0 = (i32 [%rdi], i64 [%rsi], i64 vmctx [%r14], i64 sigid [%rbx]) -> i64 [%rax] baldrdash
 
 ebb0(v0: i64):

cranelift/filetests/regalloc/reload-208.clif

@@ -11,7 +11,8 @@ target x86_64 haswell
 ; The problem was the reload pass rewriting EBB arguments on "brnz v9, ebb3(v9)"
 
 function %pr208(i64 vmctx [%rdi]) system_v {
-    gv0 = vmctx-8
+    gv1 = vmctx
+    gv0 = iadd_imm.i64 gv1, -8
     heap0 = static gv0, min 0, bound 0x5000, guard 0x0040_0000
     sig0 = (i64 vmctx [%rdi]) -> i32 [%rax] system_v
     sig1 = (i64 vmctx [%rdi], i32 [%rsi]) system_v