Add/implement isReachable()/pathsTo() for control flow #321

Is your feature request related to a problem? Please describe.
I am writing a plugin to determine if a set of constant values can propagate to a certain function with a parameter that is tagged with an LLVM IR custom attribute. I want a way to check from the entry point (or really any function) if there is a path that function to the target function.

This will allow me to select a set of paths so that I can only propogate constants along the paths to the target function

Describe the solution you'd like
Something that does this?

Describe alternatives you've considered
Maybe a backward analysis to the entry points but still need paths to get there.

Additional context
Example code

void crypt(int attribute((annotate("crypto_iv_size")) iv_size) {
}

int main() {
const int kIVSize = 8;
crypt(kIVSize);
}