[mlir] fix side effects for transform.AlternativesOp

It should have an "Allocate" effect on entry block arguments of all
regions in addition to consuming the operand.

Also relax the assertion in transform-dialect-check-uses until we can
properly support region-based control flow.

Fixes #60075.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D142200