[UnJ] Improve explicit loop count checks

Try to improve the computed counts when it has been explicitly set by a pragma
or command line option. This moves the code around, so that first call to
computeUnrollCount to get a sensible count and override that if explicit unroll
and jam counts are specified.

Also added some extra debug messages for when unroll and jamming is disabled.

Differential Revision: https://reviews.llvm.org/D50075

llvm-svn: 339501