Handle control flow edge case in JIT. Fixes rubinius#478.

See the crash test committed. The bug as because if the instruction
after a git/gif was a loop header, we wouldn't set the exception handler
of the loop body block.

vm/llvm/cfg.hpp

@@ -121,7 +121,16 @@ namespace jit {
 
     CFGBlock* add_block(int ip, bool loop=false) {
       CFGBlock* blk = find_block(ip);
-      if(blk) return blk;
+      if(blk) {
+        // If we hit a block that is the start of a loop header,
+        // be sure to set it's exception handler. These blocks are created
+        // during the first pass.
+        if(blk->loop_p()) {
+          // Inherit the current exception handler
+          blk->set_exception_handler(current_->exception_handler());
+        }
+        return blk;
+      }
 
       blk = new CFGBlock(ip, loop);
 
@@ -185,7 +194,7 @@ namespace jit {
       VMMethod::Iterator iter(stream_, stream_size_);
       for(;;) {
         if(CFGBlock* next_block = find_block(iter.position())) {
-          if(next_block->loop_p()) {
+          if(next_block->loop_p() && current_ != next_block) {
             // The handler wasn't setup originally, so we have to set it now.
             next_block->set_exception_handler(current_->exception_handler());
 

vm/test/jit/crash_goto_loop_header.rb

@@ -0,0 +1,22 @@
+# Test for a JIT bug where the while inside the if generated a goto
+# of a loop header and skipped setup up the exception handler
+# properly.
+#
+# If this doesn't crash, the bug is fixed.
+def bad
+  i = 0
+  begin
+    if i < 10
+      while i < 20
+        i += 2
+      end
+    end
+  rescue => ex
+    raise "failure is an option"
+  end
+  i
+end
+
+1000000.times do
+  bad
+end