https://pkg.go.dev/golang.org/x/tools/go/ssa#Function  does not build or expose post-dominator relationships among its constituent basic basics.

I found it essential to find post dominance in SSA functions for a lock-unlock analysis [here (https://arxiv.org/pdf/2106.01710.pdf).
The code using the post dominance can be seen here.

Constructing and exposing post dominance in an SSA Function is relatively simple. It can reuse most of the code available for dominance. I have an example buildPostdomtree() here.
We need to add APIs such as:

func (b *BasicBlock) PostDominees() []*BasicBlock
func (b *BasicBlock) PostDominates(c *BasicBlock) bool
func (f *Function) PostdomPreorder() []*BasicBlock
func (f *Function) PostdomPostorder() []*BasicBlock {
func (b *BasicBlock) Ipdom() *BasicBlock

All of these are prototyped here.