[MLIR][NFC] Minor cleanup for BufferDeallocation pass.

- Change walkReturnOperations() to be a non-template and look at block terminator
  for ReturnLike trait.
- Clarify description of validateSupportedControlFlow
- Eliminate unused argument in Backedges::recurse.
- Eliminate repeated calls to getFunction()
- Fix wording for non-SCF loop failure

Differential Revision: https://reviews.llvm.org/D106373