[test] Fix FullUnroll.ll

I believe the intention of this test added in
https://reviews.llvm.org/D71687 was to test LoopFullUnrollPass with
clang's -fno-unroll-loops, not its interaction with optnone. Loop
unrolling passes don't run under optnone/-O0.

Also added back unintentionally removed -disable-loop-unrolling from
https://reviews.llvm.org/D85578.

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D86485