Marking rs1 (memory offset base) as memory operand provides additional
semantic value to this operand that can be used by different tools
(e.g. llvm-exegesis).
This change does not affect neigther Isel nor assembler. However it
required some tweaks in tablegen compressed inst emmiter.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D136847