Specify can_branch and decode the sret instruction

sret instruction is used for returning from a trap when trap occurs in
S-mode level. Thus, the execution flow will not be sequential. During
basic block translation, the sret instruction should be considered as
can_branch instruction.

Moreover, the existing system instruction decoder does not support
decoding the sret instruction. Thus, the ir->opcode should be set
correctly to support decoding the sret instruction. The implementation
of sret instruction is simply returning false for now, the improved
implementation will be completed and tested in #438 since the sret
instruction involves privilege mode changing.

src/decode.c

@@ -827,6 +827,8 @@ static inline bool op_system(rv_insn_t *ir, const uint32_t insn)
         case 0x105: /* WFI: Wait for Interrupt */
         case 0x002: /* URET: return from traps in U-mode */
         case 0x102: /* SRET: return from traps in S-mode */
+            ir->opcode = rv_insn_sret;
+            break;
         case 0x202: /* HRET: return from traps in H-mode */
             /* illegal instruciton */
             return false;

src/decode.h

@@ -77,7 +77,7 @@ enum op_field {
     /* RISC-V Privileged Instruction */                \
     _(wfi, 0, 4, 0, ENC(rs1, rd))                      \
     _(uret, 0, 4, 0, ENC(rs1, rd))                     \
-    _(sret, 0, 4, 0, ENC(rs1, rd))                     \
+    _(sret, 1, 4, 0, ENC(rs1, rd))                     \
     _(hret, 0, 4, 0, ENC(rs1, rd))                     \
     _(mret, 1, 4, 0, ENC(rs1, rd))                     \
     /* RV32 Zifencei Standard Extension */             \