[llvm] [Support] Do not use `llvm::size` in `getLoopPreheader` (PR #94540)

[Ben Barham via llvm-commits]

bnbarham wrote:

@kazutakahirata this was a NFC commit, was there an underlying reason for the change or are you happy with reverting this part of it?

https://github.com/llvm/llvm-project/pull/94540