[mlir] Add RewriterBase::replaceAllUsesWith for Blocks.

When changing IR in a RewriterPattern, all changes must go through the
rewriter. There are several convenience functions in RewriterBase that
help with high-level modifications, such as replaceAllUsesWith for
Values, but there is currently none to do the same task for Blocks.

Reviewed By: mehdi_amini, ingomueller-net

Differential Revision: https://reviews.llvm.org/D142525