[SCEV] Compute symbolic exit count for 'and' conditions

If loop exits by condition like `A < B && X < Y`, and at least one of symbolic max
exit counts for these conditions is known, it can be used as estimate of symbolic
max exit count for whole branch. If both are known, then we can use their umin
as the estimate.

Differential Revision: https://reviews.llvm.org/D139403
Reviewed By: nikic