[MLIR] Fix getCommonBlock utility in affine analysis

Fix the hardcoded check for `FuncOp` in `getCommonBlock` utility: the
check should have been for an op that starts an affine scope. The
incorrect block returned in turn causes dependence analysis to function
incorrectly.

This change allows affine store-load forwarding to work correctly inside
any ops that start an affine scope.

Reviewed By: ftynse, dcaballe

Differential Revision: https://reviews.llvm.org/D130749