Revert reserved immediate word shifts with imm[5]!=0 #656

Job	Run time
op-program-test	2m 14s
rvgo-abigen	51s
check-optimism-version-match	28s
rvsol-lint	4s
rvgo-tests	11m 51s
rvgo-lint	2m 25s
rvsol-tests	1m 34s
	19m 27s

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