[llvm] [RegAllocFast] NFC cleanups (PR #74860)

[Matthias Braun via llvm-commits]

MatzeB wrote:

CC @LuoYuanke  I assume you changed those for-loops on purpose in https://reviews.llvm.org/D124834 ? Do you still remember why? As admittedly I cannot immediately see the reason for it, especially with that reverse-loop where added operands would not even get recognized...

https://github.com/llvm/llvm-project/pull/74860