[IndVars] Call replaceLoopPHINodesWithPreheaderValues() for already constant exits

Currently we only call replaceLoopPHINodesWithPreheaderValues() if
optimizeLoopExits() replaces the exit with an unconditional exit.
However, it is very common that this already happens as part of
eliminateIVComparison(), in which case we're leaving behind the
dead header phi.

Tweak the early bailout for already-constant exits to also call
replaceLoopPHINodesWithPreheaderValues().

Differential Revision: https://reviews.llvm.org/D129214