[LoopRotate] processLoop should return true even if it just simplified the loop latch...

[LoopRotate] processLoop should return true even if it just simplified the loop latch without making any other changes

Simplifying a loop latch changes the IR and we need to make sure the pass manager knows to invalidate analysis passes if that happened.

PR35210 discovered a case where we failed to invalidate the post dominator tree after this simplification because we no changes other than simplifying the loop latch.

Fixes PR35210.

Differential Revision: https://reviews.llvm.org/D40035

llvm-svn: 318237