[Unroll] Add an option to control complete unrolling

Add an ability to specify the max full unroll count for LoopUnrollPass pass
in pass options.

Reviewers: fhahn, fedor.sergeev
Reviewed By: fedor.sergeev
Subscribers: hiraditya, zzheng, dmgreen, llvm-commits
Differential Revision: https://reviews.llvm.org/D67701

llvm-svn: 372305