[mlir][SCF][NFC] Add helper functions to get body of scf.while

Add two new helper functions `getBeforeBody` and `getAfterBody` to be consistent with "scf.for" (`getBody`) and to show in the API that both regions have exactly one block. Also simplify some code that assumed that there can be more than one block in a region.

Differential Revision: https://reviews.llvm.org/D157860