After replacing a loop phi with the preheader value, it's usually
possible to simplify some of the using instructions, so do that as
part of replaceLoopPHINodesWithPreheaderValues().
Doing this as part of IndVars is valuable, because it may make GEPs
in the loop have constant offsets and allow the following SROA run
to succeed (as demonstrated in the PhaseOrdering test).
Differential Revision: https://reviews.llvm.org/D129293
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
These intrinsics, not the icmp+select are the canonical form nowadays,
so we might as well directly emit them.
This should not cause any regressions, but if it does,
then then they would needed to be fixed regardless.
Note that this doesn't deal with `SCEVExpander::isHighCostExpansion()`,
but that is a pessimization, not a correctness issue.
Additionally, the non-intrinsic form has issues with undef,
see https://reviews.llvm.org/D88287#2587863
