PushInventory() is currently called with a CInv object, which can be a MSG_TX or MSG_BLOCK. PushInventory() only uses the type to determine whether to add the hash to setInventoryTxToSend or vInventoryBlockToSend. Since the caller always knows what type of inventory they're pushing, the CInv is wastefully constructed and thrown away, and tx/block relay is being split out, we split the function into PushTxInventory() and PushBlockInventory().
Backport of core PR19322.