Don't emit decont of callee if unrequired

We can know it's not required when either:

* It's a QAST::WVal and the value isn't a container
* It's coming from a speshresolve; these plugins are written by the
  compiler implementor, and in the odd case where the plugin may give
  make something containerized, it's still more efficient to have the
  plugin decont it before returning it, so we can do it once, not for
  every single call

Saves around 52KB of instructions in the Rakudo CORE.setting. That's
less to interpret before optimization, and helps with inlining.

src/vm/moar/QAST/QASTOperationsMAST.nqp

@@ -1442,13 +1442,20 @@ my $call_gen := sub ($qastcomp, $op) {
     my $return_type;
     my @args := $op.list;
     if $op.name {
-        $callee := $qastcomp.as_mast($op.op eq 'callstatic'
-            ?? QAST::VM.new( :moarop('getlexstatic_o'), QAST::SVal.new( :value($op.name) ) )
-            !! QAST::Var.new( :name($op.name), :scope('lexical') ));
+        $callee := $qastcomp.as_mast(QAST::Op.new(
+            :op('decont'),
+            $op.op eq 'callstatic'
+                ?? QAST::VM.new( :moarop('getlexstatic_o'), QAST::SVal.new( :value($op.name) ) )
+                !! QAST::Var.new( :name($op.name), :scope('lexical') )));
     }
     elsif +@args {
         @args := nqp::clone(@args);
-        $callee := $qastcomp.as_mast(@args.shift(), :want($MVM_reg_obj));
+        my $callee_qast := @args.shift;
+        my $no_decont := nqp::istype($callee_qast, QAST::Op) && $callee_qast.op eq 'speshresolve'
+            || nqp::istype($callee_qast, QAST::WVal) && !nqp::iscont($callee_qast.value);
+        $callee := $qastcomp.as_mast(
+            $no_decont ?? $callee_qast !! QAST::Op.new( :op('decont'), $callee_qast ),
+            :want($MVM_reg_obj));
         if $op.op eq 'nativeinvoke' {
             $return_type := $qastcomp.as_mast(@args.shift(), :want($MVM_reg_obj));
         }
@@ -1481,14 +1488,9 @@ my $call_gen := sub ($qastcomp, $op) {
         handle_arg($_, $qastcomp, @ins, @arg_regs, @arg_flags, @arg_kinds);
     }
 
-    # Decontainerize the callee.
+    # Release the callee's result register.
     my $regalloc := $*REGALLOC;
-    my $decont_reg := $regalloc.fresh_register($MVM_reg_obj);
-    push_op(@ins, 'decont', $decont_reg, $callee.result_reg);
-
-    # Release the callee's result register and the decont register.
     $regalloc.release_register($callee.result_reg, $MVM_reg_obj);
-    $regalloc.release_register($decont_reg, $MVM_reg_obj);
 
     # Release each arg's result register
     my $arg_num := 0;
@@ -1517,7 +1519,7 @@ my $call_gen := sub ($qastcomp, $op) {
 
     # Generate call.
     nqp::push(@ins, MAST::Call.new(
-        :target($decont_reg),
+        :target($callee.result_reg),
         :flags(@arg_flags),
         |@arg_regs,
         |%result,