[MLIR] Update the FunctionAndBlockSignatureConverter and NonVoidToVoidReturnOpConvert...

[MLIR] Update the FunctionAndBlockSignatureConverter and NonVoidToVoidReturnOpConverter of Buffer Assignment

Making these two converters more generic. FunctionAndBlockSignatureConverter now
moves only memref results (after type conversion) to the function argument and
keeps other legal function results unchanged. NonVoidToVoidReturnOpConverter is
renamed to NoBufferOperandsReturnOpConverter. It removes only the buffer
operands from the operands of the converted ReturnOp and inserts CopyOps to copy
each buffer to the target function argument.

Differential Revision: https://reviews.llvm.org/D79329