Cleanup ConstantExpr handling:

 * Correctly delete TypeHandles in AsmParser.  In addition to not leaking
   memory, this prevents a bug that could have occurred when a type got
   resolved that the constexpr was using
 * Check for errors in the AsmParser instead of hitting assertion failures
   deep in the code
 * Simplify the interface to the ConstantExpr class, removing unneccesary
   parameters to the ::get* methods.
 * Rename the 'getelementptr' version of ConstantExpr::get to
   ConstantExpr::getGetElementPtr

llvm-svn: 3160