This patch extends the LoopUnroll pass to be able to unroll loops

This patch extends the LoopUnroll pass to be able to unroll loops
with unknown trip counts. This is left off by default, and a
command-line option enables it. It also begins to separate loop
unrolling into a utility routine; eventually it might be made usable
from other passes.

It currently works by inserting conditional branches between each
unrolled iteration, unless it proves that the trip count is a
multiple of a constant integer > 1, which it currently only does in
the rare case that the trip count expression is a Mul operator with
a ConstantInt operand. Eventually this information might be provided
by other sources, for example by a pass that peels/splits the loop
for this purpose.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36990 91177308-0d34-0410-b5e6-96231b3b80d8