convert formal argument to local variable in LLVM

I am doing a project on LLVM.

I'm trying to make a pass:

For a function, if the arguments(parameters) are constants, it will make a new function that the constants are converted to a local variable, and deleted from arguments.

For example:

void main(){
    c=foo(a,b);
    d=foo(a,100);
}

double foo(x,y){return x+y;}

will get optimized into:

void main(){
    c=foo(a,b);
    d=foo1(a);
}  

double foo(x,y){return x+y;}
double foo1(x){
    int y=100;
    return x+y;
}

To do this, I'm trying to make a new local variable and then using replaceAllUsesWith function.

But I can't seem to allocate a new local variable.

I did try defining new Value* but when I assign it to the argument, it becomes same pointer as the argument, which makes replaceAllUsesWith function useless.

Is there a way to allocate (in the memory) a new local variable in type of Value*?