[SCEV] ] If RHS >= Start, simplify (Start smax RHS) to RHS for trip counts.

This is the max version of D85046.

This change causes binary changes in 44 out of 237 benchmarks (out of
MultiSource/SPEC2000/SPEC2006)

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D85189