Transform end of loop conditions to min_expr

The fundamental idea is to convert an end of loop computation of the form i < x && i < y into i < min (x, y) especially where x and y are invariant. Doing this before the vectorizer in the optimization pipeline would mean that such loops can also be vectorized.

A couple of suggestions for this were to do it in tree-ssa-fwprop.c or tree-ssa-ifcombine.c depending on the form of the IR that we see.

http://gcc.gnu.org/ml/gcc/2012-05/msg00009.html

http://gcc.gnu.org/ml/gcc/2012-04/msg00766.html