[mlir] check whether region and block visitors are interrupted

The visitor functions for `Region` and `Block` types did not always
check the value returned by recursive calls.  This caused the top-level
visitor invocation to return `WalkResult::advance()` even if one or more
recursive invocations returned `WalkResult::interrupt()`.  This patch
fixes the problem by check if any recursive call is interrupted, and if
so, return `WalkResult::interrupt()`.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D129718