Tweak the LICM code to reuse the first sub-loop instead of creating a new one

LICM starts with an *empty* AST, and then merges in each sub-loop. While the
add code is appropriate for sub-loop 2 and up, it's utterly unnecessary for
sub-loop 1. If the AST starts off empty, we can just clone/move the contents
of the subloop into the containing AST.

Reviewed-by: Philip Reames <listmail@philipreames.com>

Differential Revision: http://reviews.llvm.org/D16753

llvm-svn: 260892