[mlir][linalg][bufferize] Support scf::IfOp

This commit adds support for scf::IfOp to comprehensive bufferization. Support is currently limited to cases where both branches yield tensors that bufferize to the same buffer.

To keep the analysis simple, scf::IfOp are treated as memory writes for analysis purposes, even if no op inside any branch is writing. (scf::ForOps are handled in the same way.)

Differential Revision: https://reviews.llvm.org/D111929