[AMDGPU] Use correct operand order for shifts (#68299)

In a special case in frame index elimination (when the offset is 0), we
generate either a S_LSHR_B32 or a V_LSHRREV_B32 using the same code.
However, they don't expect their operands in the same order - S_LSHR_B32
takes the value to be shifted first and then the shift amount, whereas
V_LSHRREV_B32 has the operands reversed (hence the REV in its name).
Update the code & tests to take this into account. Also remove an
outdated comment (this code is definitely reachable now that non-entry
functions no longer have a fixed emergency scavenge slot).