[Unroll] Do NOT unroll a loop with small runtime upperbound

For a runtime loop if we can compute its trip count upperbound:

Don't unroll if:
1. loop is not guaranteed to run either zero or upperbound iterations; and
2. trip count upperbound is less than UnrollMaxUpperBound
Unless user or TTI asked to do so.

If unrolling, limit unroll factor to loop's trip count upperbound.

Differential Revision: https://reviews.llvm.org/D62989

Change-Id: I6083c46a9d98b2e22cd855e60523fdc5a4929c73
llvm-svn: 373017