memory_dff - merge input/output DFFs into memory read ports

yosys> help memory_dff
memory_dff [-no-rw-check] [selection]
This pass detects DFFs at memory read ports and merges them into the memory
port. I.e. it consumes an asynchronous memory port and the flip-flops at its
interface and yields a synchronous memory port.
marks all recognized read ports as "return don't-care value on
read/write collision" (same result as setting the no_rw_check
attribute on all memories).