[RLEV] Pick a correct insert point when incoming instruction is itself a phi node

This fixes https://github.com/llvm/llvm-project/issues/57336. It was exposed by a recent SCEV change, but appears to have been a long standing issue.

Note that the whole insert into the loop instead of a split exit edge is slightly contrived to begin with; it's there solely because IndVarSimplify preserves the CFG.

Differential Revision: https://reviews.llvm.org/D132571