[mlir] Finish removing FunctionPass

FunctionPass has been deprecated in favor of OperationPass<FuncOp>
for a few weeks, and this commit finished the deprecation with deletion.
The only difference between the two is that FunctionPass filters out function
declarations. When updating references to FunctionPass, ensure that
the pass either can handle declarations or explicitly add in filtering.

See https://llvm.discourse.group/t/functionpass-deprecated-in-favor-of-operationpass-funcop

Differential Revision: https://reviews.llvm.org/D118735