8328085: C2: Use after free in PhaseChaitin::Register_Allocate()

Reviewed-by: thartmann, mdoerr

src/hotspot/share/opto/postaloc.cpp

@@ -402,7 +402,6 @@ bool PhaseChaitin::eliminate_copy_of_constant(Node* val, Node* n,
 // as they get encountered with the merge node and keep adding these defs to the merge inputs.
 void PhaseChaitin::merge_multidefs() {
   Compile::TracePhase tp(_t_mergeMultidefs);
-  ResourceMark rm;
   // Keep track of the defs seen in registers and collect their uses in the block.
   RegToDefUseMap reg2defuse(_max_reg, _max_reg, RegDefUse());
   for (uint i = 0; i < _cfg.number_of_blocks(); i++) {