[SCEV] Use fact that B >u 0 for A <u B in applyLoopGuards.

If LHS <u RHS holds, RHS should be guaranteed to be > 0. By using
using 'umax(RHS, 1) -1' instead of 'RHS - 1' the results in
applyLoopGuards can be improved in some cases.

Note that the TODO for the tests mentioned the max BTC being 11, but
unless I am missing something 10 should be correct.

https://alive2.llvm.org/ce/z/44nP7F

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D126503