af49bed933
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