[NFCI][ValueTracking] getUnderlyingObject(): assert that no cycles are encountered

Jeroen Dobbelaere in
https://lists.llvm.org/pipermail/llvm-dev/2021-March/149206.html
is reporting that this function can end up in an endless loop
when called from SROA w/ full restrict patches.

For now, simply ensure that such problems are caught earlier/easier.