Replace p6recont_ro C ext op with a desugar

This results in slightly larger code (though this is not a commonly
occurring thing), but means that spesh can see into it further and
hopefully better optimize it.

This also fixes the longstanding bug with recont_ro mishandling a
Proxy. Sadly, that in turn regresses spectests.

src/Perl6/Actions.nqp

@@ -709,6 +709,42 @@ register_op_desugar('p6scalarfromdesc', -> $qast {
         )
     )
 });
+register_op_desugar('p6recont_ro', -> $qast {
+    my $result := QAST::Node.unique('result');
+    my $Scalar := QAST::WVal.new( :value(nqp::gethllsym('perl6', 'Scalar')) );
+    QAST::Stmt.new(
+        QAST::Op.new(
+            :op('bind'),
+            QAST::Var.new( :name($result), :scope('local'), :decl('var') ),
+            $qast[0]
+        ),
+        QAST::Op.new(
+            :op('if'),
+            QAST::Op.new(
+                :op('if'),
+                QAST::Op.new(
+                    :op('isconcrete_nd'),
+                    QAST::Var.new( :name($result), :scope('local') )
+                ),
+                QAST::Op.new(
+                    :op('isrwcont'),
+                    QAST::Var.new( :name($result), :scope('local') )
+                )
+            ),
+            QAST::Op.new(
+                :op('p6bindattrinvres'),
+                QAST::Op.new( :op('create'), $Scalar ),
+                $Scalar,
+                QAST::SVal.new( :value('$!value') ),
+                QAST::Op.new(
+                    :op('decont'),
+                    QAST::Var.new( :name($result), :scope('local') )
+                )
+            ),
+            QAST::Var.new( :name($result), :scope('local') )
+        )
+    )
+});
 
 sub can-use-p6forstmt($block) {
     my $past_block := $block.ann('past_block');

src/vm/moar/Perl6/Ops.nqp

@@ -47,9 +47,6 @@ MAST::ExtOpRegistry.register_extop('p6var',
 MAST::ExtOpRegistry.register_extop('p6reprname',
     $MVM_operand_obj   +| $MVM_operand_write_reg,
     $MVM_operand_obj   +| $MVM_operand_read_reg);
-MAST::ExtOpRegistry.register_extop('p6recont_ro',
-    $MVM_operand_obj   +| $MVM_operand_write_reg,
-    $MVM_operand_obj   +| $MVM_operand_read_reg);
 MAST::ExtOpRegistry.register_extop('p6decontrv',
     $MVM_operand_obj   +| $MVM_operand_write_reg,
     $MVM_operand_obj   +| $MVM_operand_read_reg);
@@ -108,7 +105,6 @@ $ops.add_hll_moarop_mapping('perl6', 'p6box_i', 'p6box_i');
 $ops.add_hll_moarop_mapping('perl6', 'p6box_n', 'p6box_n');
 $ops.add_hll_moarop_mapping('perl6', 'p6box_s', 'p6box_s');
 $ops.add_hll_moarop_mapping('perl6', 'p6box_u', 'p6box_u');
-$ops.add_hll_moarop_mapping('perl6', 'p6recont_ro', 'p6recont_ro');
 $ops.add_hll_op('perl6', 'p6store', -> $qastcomp, $op {
     my @ops;
     my $cont_res  := $qastcomp.as_mast($op[0], :want($MVM_reg_obj));

src/vm/moar/ops/perl6_ops.c

@@ -217,28 +217,6 @@ static void p6bool_discover(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshIns
 #endif
 }
 
-static MVMuint8 s_p6recont_ro[] = {
-    MVM_operand_obj | MVM_operand_write_reg,
-    MVM_operand_obj | MVM_operand_read_reg,
-};
-static void p6recont_ro(MVMThreadContext *tc, MVMuint8 *cur_op) {
-    MVMObject *check = GET_REG(tc, 2).o;
-    if (IS_CONCRETE(check) && STABLE(check)->container_spec == Rakudo_containers_get_scalar()) {
-        MVMObject *desc = ((Rakudo_Scalar *)check)->descriptor;
-        if (!MVM_is_null(tc, desc) && ((Rakudo_ContainerDescriptor *)desc)->rw) {
-            /* We have an rw container; re-containerize it. */
-            MVMROOT(tc, check, {
-                MVMObject *result = MVM_repr_alloc_init(tc, Scalar);
-                MVM_ASSIGN_REF(tc, &(result->header), ((Rakudo_Scalar *)result)->value,
-                    ((Rakudo_Scalar *)check)->value);
-                GET_REG(tc, 0).o = result;
-            });
-            return;
-        }
-    }
-    GET_REG(tc, 0).o = check;
-}
-
 /* The .VAR operation. Wraps in an outer Scalar container so we can actually
  * operate on the underlying Scalar, if we have a container. */
 static MVMuint8 s_p6var[] = {
@@ -701,7 +679,6 @@ MVM_DLL_EXPORT void Rakudo_ops_init(MVMThreadContext *tc) {
     MVM_ext_register_extop(tc, "p6box_u",  p6box_u, 2, s_p6box_u, NULL, p6box_u_discover, MVM_EXTOP_PURE | MVM_EXTOP_ALLOCATING);
     MVM_ext_register_extop(tc, "p6settypes",  p6settypes, 1, s_p6settypes, NULL, NULL, 0);
     MVM_ext_register_extop(tc, "p6bool",  p6bool, 2, s_p6bool, NULL, p6bool_discover, MVM_EXTOP_PURE);
-    MVM_ext_register_extop(tc, "p6recont_ro",  p6recont_ro, 2, s_p6recont_ro, NULL, NULL, MVM_EXTOP_PURE);
     MVM_ext_register_extop(tc, "p6var",  p6var, 2, s_p6var, NULL, NULL, MVM_EXTOP_PURE | MVM_EXTOP_ALLOCATING);
     MVM_ext_register_extop(tc, "p6reprname",  p6reprname, 2, s_p6reprname, NULL, p6reprname_discover, MVM_EXTOP_PURE | MVM_EXTOP_ALLOCATING);
     MVM_ext_register_extop(tc, "p6decontrv",  p6decontrv, 2, s_p6decontrv, p6decontrv_spesh, NULL, MVM_EXTOP_PURE);