add a -LHS command line option to parser-test, and use it in
test/Extractor/illegal-sext.ll

make infer() take an InstOrigin * instead of an InstOrigin & to that clients
lacking an InstOrigin can still use it

modify souper-check to optionally infer a RHS for a replacement lacking it

add a new test infer.opt that runs souper-check in infer mode, makes sure a RHS
is synthesized, and makes sure the concatenated LHS and RHS both parse and
check
style cleanup