Update comment about is_leaf

Update comment about is_leaf

crtl->is_leaf has a comment above it referring to "Local Register
Allocation". However this is set by IRA and not LRA since the meaning
of Local Register Allocator in GCC has changed quite drastically since
1999 when this comment was introduced above the variable
current_function_is_leaf. From memory gcc in those days had "local"
and "global" register allocation - however all of this has changed
quite a bit drastically since.

This patch merely updates the comment as I did some head scratching
this morning about how this could be set before Local Register
Allocator especially given the comment in ira.c as below.

 /* Determine if the current function is a leaf before running IRA
     since this can impact optimizations done by the prologue and
     epilogue thus changing register elimination offsets.  */
  crtl->is_leaf = leaf_function_p ();

From-SVN: r249223