[LoopUnroll] Disable loop unroll when user explicitly asks for unroll-and-jam

If a loop isn't forced to be unrolled, we want to avoid unrolling it when there
is an explicit unroll-and-jam pragma. This is to prevent automatic unrolling
from interfering with the user requested transformation.
Missed adding the testcase in earlier commit.

Differential Revision: https://reviews.llvm.org/D114886