Backport IsValid optimization to 3.8

[dbaston]

Some optimizations to IsValidOp where recently committed to master. One of these, 7ed0c824b76e2127a16597546e507/git is very small and of high enough benefit that I think it's worth backporting to 3.8.

[Daniel Baston <dbaston@…>]

In c2a683d/git:

Avoid MonotoneChain self-compare in SimpleMCSweepLineIntersector

This is a hotspot in validity profiling. This commit reduces the runtime
of GEOSisValid from 3.7s to 2.0s for the 5,489 polygons in the
HydroBASINS level 5 dataset, and from 610ms to 360ms for the individual
polygon components of the boundary of Australia in the GADM country
dataset.

Fixes #1008