This package started as a copy of golang.org/x/tools/go/ssa, imported from an unknown commit in 2016.
It has since been heavily modified to match our own needs in an IR.
The changes are too many to list here, and it is best to consider this package independent of go/ssa.

Upstream changes still get applied when they address bugs in portions of code we have inherited.

The last upstream commit we've looked at was:
915f6209478fe61eb90dbe155a8a1c58655b931f

