Reading global variable is in general not SideEffectFree

* Global variables can have hooks with rb_define_hooked_variable()
  and can do arbitrary things.
* $! (and $ERROR_INFO and $@) can access the backtrace of the exception,
  so they are not safe for the BACKTRACES_OMIT_UNUSED optimization.

Author: eregon

src/main/java/org/truffleruby/parser/BodyTranslator.java

@@ -2840,11 +2840,7 @@ public RubyNode visitRescueNode(RescueParseNode node) {
         boolean canOmitBacktrace = false;
 
         if (context.getOptions().BACKTRACES_OMIT_UNUSED && rescueBody != null &&
-                rescueBody.getBodyNode() instanceof SideEffectFree /* allow `expression rescue $!` pattern */ &&
-                (!(rescueBody.getBodyNode() instanceof GlobalVarParseNode) ||
-                        (!((GlobalVarParseNode) rescueBody.getBodyNode()).getName().equals("$!") &&
-                                !((GlobalVarParseNode) rescueBody.getBodyNode()).getName().equals("$ERROR_INFO"))) &&
-                rescueBody.getOptRescueNode() == null) {
+                rescueBody.getBodyNode() instanceof SideEffectFree && rescueBody.getOptRescueNode() == null) {
             canOmitBacktrace = true;
         }
 

src/main/java/org/truffleruby/parser/ast/GlobalVarParseNode.java

@@ -40,7 +40,7 @@
 /**
  * access to a global variable.
  */
-public class GlobalVarParseNode extends ParseNode implements INameNode, SideEffectFree {
+public class GlobalVarParseNode extends ParseNode implements INameNode {
     private String name;
 
     public GlobalVarParseNode(SourceIndexLength position, String name) {