Issues found PVS studio static analyzer. LoopUnrollAndJam.cpp

Main issue:
https://github.com/llvm/llvm-project/issues/169959
20. Minor misoptimization. The PVS-Studio warning: V791 The initial value of the index in the nested loop equals 'I'. Perhaps, 'I + 1' should be used instead. LoopUnrollAndJam.cpp 793

Start with LoopUnrollAndJam.cpp around line 793 and read the related issue #169959 for context on the PVS-Studio V791 warning. Determine whether the warning represents a real misoptimization, then confirm the intended loop behavior and add or update coverage as appropriate; done means the reported issue is addressed and the relevant compiler tests pass.

Written by the indexing model from the issue text.