unifying apis that require sign flag.

[LaoLittle]

Hi.

When I was using inkwell for code generation, I found that some operations (like div/rem) are divided into two functions
because they require a sign flag to work.

And then I was using shift apis and was trying to figure out where is the build_right_shift_signed function, but there's only one function called build_right_shift which takes an argument to determine whether the operation is sign extended.

Describe the solution you'd like

Make the build_int_signed_op to build_int_op which takes an argument called sign_extend