[llvm-branch-commits] [NFCI][PromoteMem2Reg] Don't handle the first successor out of order (PR #142464)

[Vitaly Buka via llvm-branch-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/142464

Additionally handler successors in regular order, so that two successors
cases are handled exactly as before, so we need to less tests.

Additionally reverse order here results in more
natural 'phi' arguments list order.