Add test for constant RHS in add

Author: lukel97

llvm/test/CodeGen/RISCV/add_sext_shl_constant.ll

@@ -333,3 +333,24 @@ define i64 @add_shl_moreOneUse_sh4add(i64 %x) {
   %add = add i64 %mul, %or
   ret i64 %add
 }
+
+define i64 @add_shl_rhs_constant(i64 %x, i64 %y) {
+; NO-ZBA-LABEL: add_shl_rhs_constant:
+; NO-ZBA:       # %bb.0:
+; NO-ZBA-NEXT:    add a0, a0, a1
+; NO-ZBA-NEXT:    slli a0, a0, 3
+; NO-ZBA-NEXT:    ret
+;
+; ZBA-LABEL: add_shl_rhs_constant:
+; ZBA:       # %bb.0:
+; ZBA-NEXT:    add a0, a0, a1
+; ZBA-NEXT:    addi a0, a0, 1
+; ZBA-NEXT:    slli a0, a0, 3
+; ZBA-NEXT:    addi a0, a0, -8
+; ZBA-NEXT:    ret
+  %a = add i64 %x, 1
+  %b = add i64 %y, %a
+  %c = shl i64 %b, 3
+  %d = add i64 %c, -8
+  ret i64 %d
+}