8311862: RISC-V: small improvements to shift immediate instructions

Reviewed-by: luhenry, fjiang, fyang

Author: Ilya Gavrilin

src/hotspot/cpu/riscv/assembler_riscv.hpp

@@ -2787,7 +2787,13 @@ enum Nf {
       c_slli(Rd, shamt);                                                                     \
       return;                                                                                \
     }                                                                                        \
-    _slli(Rd, Rs1, shamt);                                                                   \
+    if (shamt != 0) {                                                                        \
+      _slli(Rd, Rs1, shamt);                                                                 \
+    } else {                                                                                 \
+      if (Rd != Rs1) {                                                                        \
+        addi(Rd, Rs1, 0);                                                                    \
+      }                                                                                      \
+    }                                                                                        \
   }
 
   INSN(slli);
@@ -2802,7 +2808,13 @@ enum Nf {
       C_NAME(Rd, shamt);                                                                     \
       return;                                                                                \
     }                                                                                        \
-    NORMAL_NAME(Rd, Rs1, shamt);                                                             \
+    if (shamt != 0) {                                                                        \
+      NORMAL_NAME(Rd, Rs1, shamt);                                                           \
+    } else {                                                                                 \
+      if (Rd != Rs1) {                                                                        \
+        addi(Rd, Rs1, 0);                                                                    \
+      }                                                                                      \
+    }                                                                                        \
   }
 
   INSN(srai, c_srai, _srai);