Fix codegen for modulo operation

There was a regression with the modulo operation. It turns out the srem
instruction does a signed division under the hood. Somewhat obvious
after the fact but easy to miss.

Example of previously broken script:

    BEGIN
    {
      @start = 0;
    }

    interval:ms:1
    {
      if ((@start % 200) == 0) {
        ++@start;
      }
    }

Compile error:

    Error: Unsupport signed division for DAG: 0x11d46d8: i64 = sdiv
    0x11d4d58, Constant:i64<200>Please convert to unsigned div/mod.
    ...

src/ast/codegen_llvm.cpp

@@ -875,7 +875,11 @@ void CodegenLLVM::visit(Binop &binop)
       case bpftrace::Parser::token::MUL:   expr_ = b_.CreateMul    (lhs, rhs); break;
       case bpftrace::Parser::token::DIV:   expr_ = b_.CreateUDiv   (lhs, rhs); break;
       case bpftrace::Parser::token::MOD: {
-        expr_ = do_signed ? b_.CreateSRem(lhs, rhs) : b_.CreateURem(lhs, rhs);
+        // Always do an unsigned modulo operation here even if `do_signed`
+        // is true. bpf instruction set does not support signed divison.
+        // We already warn in the semantic analyser that signed modulo can
+        // lead to undefined behavior (because we will treat it as unsigned).
+        expr_ = b_.CreateURem(lhs, rhs);
         break;
       }
       case bpftrace::Parser::token::BAND:  expr_ = b_.CreateAnd    (lhs, rhs); break;

src/ast/semantic_analyser.cpp

@@ -895,7 +895,8 @@ void SemanticAnalyser::visit(Binop &binop)
       //
       // SDIV is not implemented for bpf. See Documentation/bpf/bpf_design_QA
       // in kernel sources
-      if (binop.op == bpftrace::Parser::token::DIV) {
+      if (binop.op == bpftrace::Parser::token::DIV ||
+          binop.op == bpftrace::Parser::token::MOD) {
         // Convert operands to unsigned if possible
         if (lsign && left->is_literal && get_int_literal(left) >= 0)
           left->type.is_signed = lsign = false;