shouldUseEVM could be slightly optimized

[technicallyty]

shoudlUseEVM function (mempool/iterator.go:142) calls:

	nextEVMTx, evmFee := i.getNextEVMTx()
	nextCosmosTx, cosmosFee := i.getNextCosmosTx()

however, one of it's callers, getPreferredTransaction, already has these variables loaded. another one of it's callers: advanceCurrentIterator doesn't.

i think we could make a small perf optimization by allowing getPreferredTransactionto pass in its values.

[linear]

EVM-418