[LSR] Generalize one aspect of terminator folding (recently introduced in D132443)

There's no need to require the start value to come directly from the loop predecessor.  This was sometimes covering up a latent miscompile in this off-by-default option, but the miscompile needs fixed anyways and the issue has been raised on the original review.

Differential Revision: https://reviews.llvm.org/D142240