The expr_ of a PositionalParameter is a pointer to a buffer, and an
offset can be added to it (e.g., `str($1 + 1)`). Cast expr_ to int so
that llvm does not complain about type mismatches when operating addition.

This fixes the following error reported by AddressSanitizer.

```
% sudo ./src/bpftrace -e 'BEGIN { printf("%s", str($1 + 1)); exit(); }' hello
AddressSanitizer:DEADLYSIGNAL
=================================================================
==3979==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000f8b (pc 0x7f1947a755ba bp 0x60700000def0 sp 0x7ffc07b486b0 T0)
==3979==The signal is caused by a READ memory access.
==3979==Hint: address points to the zero page.
    #0 0x7f1947a755ba in llvm::Constant::isNullValue() const (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xa735ba)
    bpftrace#1 0x7f1948826177 in llvm::LazyValueInfo::getPredicateAt(unsigned int, llvm::Value*, llvm::Constant*, llvm::Instruction*) (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x1824177)
    bpftrace#2 0x7f194841e4a8  (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x141c4a8)
    bpftrace#3 0x7f1948420049  (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x141e049)
    bpftrace#4 0x7f1947b13ed5 in llvm::FPPassManager::runOnFunction(llvm::Function&) (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xb11ed5)
    bpftrace#5 0x7f194879ca90  (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x179aa90)
    bpftrace#6 0x7f1947b145ff in llvm::legacy::PassManagerImpl::run(llvm::Module&) (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xb125ff)
    bpftrace#7 0x851bbe in bpftrace::ast::CodegenLLVM::optimize() /home/ubuntu/work/bpftrace/bpftrace/src/ast/codegen_llvm.cpp:2720:6
    bpftrace#8 0x791416 in main /home/ubuntu/work/bpftrace/bpftrace/src/main.cpp:745:10
    bpftrace#9 0x7f1945cc8b96 in __libc_start_main /build/glibc-2ORdQG/glibc-2.27/csu/../csu/libc-start.c:310
    bpftrace#10 0x4d1c79 in _start (/home/ubuntu/work/bpftrace/bpftrace/build_dev/src/bpftrace+0x4d1c79)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/usr/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xa735ba) in llvm::Constant::isNullValue() const
==3979==ABORTING
```

Extend the lifetime of a buffer of a PositionalParameter in the binop
so that the buffer live until str() is accepted.

Example:

```
% ./src/bpftrace -e 'BEGIN { printf("%s", str($1+1)); exit(); }' hello
```

before
```
  [...]
  %str1 = alloca [6 x i8], align 1
  [...]
  %4 = getelementptr inbounds [6 x i8], [6 x i8]* %str1, i64 0, i64 0
  [...]
  %5 = zext [6 x i8]* %str1 to i64
  %6 = add nuw nsw i64 %5, 1
  call void @llvm.lifetime.end.p0i8(i64 -1, i8* nonnull %4)
  %probe_read_kernel_str = call i64 inttoptr (i64 115 to i64 ([64 x i8]*, i32, i64)*)([64 x i8]* nonnull %str, i32 64, i64 %6)
```

after

```
  %probe_read_kernel_str = call i64 inttoptr (i64 115 to i64 ([64 x i8]*, i32, i64)*)([64 x i8]* nonnull %str, i32 64, i64 %6)
  call void @llvm.lifetime.end.p0i8(i64 -1, i8* nonnull %4)
```