Add encoding tests for RV32.
Add encoding tests for RV32.

The 32-bit CPU mode uses a different encoding for iadd_imm.i32, and 64-bit
instructions are not supported.

sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne