* Change ExpressionConvertableToType to more closely match map behavior of

  ConvertExpressionToType
* Make ValueHandle's remove instruction from maps when they are deleted so that
  no false map hits occur if a subsequent instruction is allocated to the same
  space in memory.  This was a VERY VERY VERY EVIL NASTY BUG to track down. :-P

llvm-svn: 1288