Remove unnecessary static_cast to avoid wrong downcast
Remove unnecessary static_cast to avoid wrong downcast

UBSan reports the following error.

```
% sudo ./src/bpftrace -e 'BEGIN { @ = lhist(pid, 0, 100, 1); }'
/work/bpftrace2/src/ast/codegen_llvm.cpp:462:26: runtime error: downcast of address 0x611000002d40 which does not point to an object of type 'bpftrace::ast::Integer'
0x611000002d40: note: object is of type 'bpftrace::ast::Builtin'
 08 00 80 7e  a8 a6 39 01 00 00 00 00  00 00 00 00 00 00 00 00  01 00 00 00 0d 00 00 00  00 00 00 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'bpftrace::ast::Builtin'
```

There is no need to static cast to Integer, so simply remove them.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace