[LSR] Fold terminating condition to other IV when possible

[LSR] Fold terminating condition to other IV when possible

When the IV is only used by the terminating condition (say IV-A) and the loop
has a predictable back-edge count and we have another IV (say IV-B) that is an
affine add recursion, we will be able to calculate the terminating value of
IV-B in the loop pre-header. This patch adds attempts to replace IV-B as the
new terminating condition and remove IV-A. It is safe to do so since IV-A is
only used as the terminating condition.

This transformation is suitable to be appended after LSR as it may optimize the
loop into the situation mentioned above. The transformation can reduce number of
IV-s in the loop by one.

A cli option `lsr-term-fold` is added and default disabled.

Reviewed By: mcberg2021, craig.topper

Differential Revision: https://reviews.llvm.org/D132443