Use an extra shift-by-1 instead of a compare and select to handle the
shift-by-zero case. This sometimes saves one instruction (if the compare
couldn't be combined with a previous instruction). It also works better
on targets that don't have good select instructions.
Note that currently this change doesn't affect most targets because
expandFunnelShift is not used because funnel shift intrinsics are
lowered early in SelectionDAGBuilder. But there is work afoot to change
that; see D77152.