[SCEV] Introduce bulk umin creation utilities

Add new umin creation method which accepts a list of operands.

SCEV does not represents umin which is required in getExact, so
it transforms umin to umax with not. As a result the transformation of
tree of max to max with several operands does not work.
We just use the new introduced method for creation umin from several operands.

Reviewers: sanjoy, mkazantsev
Reviewed By: sanjoy
Subscribers: javed.absar, llvm-commits
Differential Revision: https://reviews.llvm.org/D46047

llvm-svn: 331015