Fix full unrolling with new pass manager.

Fix full unrolling with new pass manager.

Last we looked at this and couldn't come up with a reason to change
it, but with a pragma for full loop unrolling we bypass every other
loop unroll and then fail to fully unroll a loop when the pragma is set.

Move the OnlyWhenForced out of the check and into the initialization
of the full unroll pass in the new pass manager. This doesn't show up
with the old pass manager.

Add a new option to opt so that we can turn off loop unrolling
manually since this is a difference between clang and opt.

Tested with check-clang and check-llvm.