Custom legalization for global_addr.

The code to compute the address of a global variable depends on the kind
of variable, so custom legalization is required.

- Add a legalizer::globalvar module which exposes an
  expand_global_addr() function. This module is likely to grow as we add
  more types of global variables.
- Add a ArgumentPurpose::VMContext enumerator. This is used to represent
  special 'vmctx' arguments that are used as base pointers for vmctx
  globals.

cranelift/docs/langref.rst

@@ -373,7 +373,7 @@ calling convention:
     retlist   : arglist
     arg       : type [argext] [argspecial]
     argext    : "uext" | "sext"
-    argspecial: "sret" | "link" | "fp" | "csr"
+    argspecial: "sret" | "link" | "fp" | "csr" | "vmctx"
     callconv  : `string`
 
 Arguments and return values have flags whose meaning is mostly target

cranelift/filetests/isa/intel/legalize-memory.cton

@@ -0,0 +1,29 @@
+; Test the legalization of memory objects.
+test legalizer
+set is_64bit
+isa intel
+
+; regex: V=v\d+
+
+function %vmctx(i64 vmctx) -> i64 {
+    gv1 = vmctx-16
+
+ebb1(v1: i64):
+    v2 = global_addr.i64 gv1
+    ; check: $v2 = iadd_imm $v1, -16
+    return v2
+    ; check: return $v2
+}
+
+function %deref(i64 vmctx) -> i64 {
+    gv1 = vmctx-16
+    gv2 = deref(gv1)+32
+
+ebb1(v1: i64):
+    v2 = global_addr.i64 gv2
+    ; check: $(a1=$V) = iadd_imm $v1, -16
+    ; check: $(p1=$V) = load.i64 $a1
+    ; check: $v2 = iadd_imm $p1, 32
+    return v2
+    ; check: return $v2
+}