[mlir][Transforms][NFC] Make signature conversion more efficient (#83922)

During block signature conversion, a new block is inserted and ops are
moved from the old block to the new block. This commit changes the
implementation such that ops are moved in bulk (`splice`) instead of
one-by-one; that's what `splitBlock` is doing.

This also makes it possible to pass the new block argument types
directly to `createBlock` instead of using `addArgument` (which bypasses
the rewriter). This doesn't change anything from a technical point of
view (there is no rewriter API for adding arguments at the moment), but
the implementation reads a bit nicer.