Remove erroneous decont op in chain compilation

Removing entirely superfluous decont operation included when compiling
MAST for the chain op. Not only was this decont not doing any good, but
in some scenarios it would result in error (see
[Github Issue 1886](rakudo/rakudo#1886))
although it's unclear to me why it's sometimes a useless addition and
sometimes an explicit error.

src/vm/moar/QAST/QASTOperationsMAST.nqp

@@ -639,7 +639,7 @@ my $chain_gen := sub ($qastcomp, $op) {
 
     # Check if callee sub in name, if not first child is callee, not arg
     my $arg_idx;
-    my &get_arg_idx := -> $cq { nqp::if( $cq.name, 0, 1 ) };
+    my &get_arg_idx := -> $cq { $cq.name ?? 0 !! 1 };
 
     while nqp::istype($cqast, QAST::Op)
     && ($cqast.op eq 'chain' || $cqast.op eq 'chainstatic') {
@@ -651,7 +651,6 @@ my $chain_gen := sub ($qastcomp, $op) {
     my @ops;
     my $regalloc := $*REGALLOC;
     my $res_reg  := $regalloc.fresh_register($MVM_reg_obj);
-    my $decont_reg  := $regalloc.fresh_register($MVM_reg_obj);
     my $endlabel := MAST::Label.new();
 
     $cqast := nqp::pop(@clist);
@@ -682,10 +681,8 @@ my $chain_gen := sub ($qastcomp, $op) {
             :result($res_reg),
             $acomp.result_reg, $bcomp.result_reg
         ));
-        push_op(@ops, 'decont', $decont_reg, $callee.result_reg);
 
         $regalloc.release_register($callee.result_reg, $MVM_reg_obj);
-        $regalloc.release_register($decont_reg, $MVM_reg_obj);
         $regalloc.release_register($acomp.result_reg, $MVM_reg_obj);
 
         if @clist {