[NNC] Fuse identical conditions in simplifier (#44886)
Summary:
Adds a pass to the IR Simplifier which fuses together the bodies of Cond statements which have identical conditions. e.g.
```
if (i < 10) {
do_thing_1;
} else {
do_thing_2;
}
if (i < 10) {
do_thing_3;
}
```
is transformed into:
```
if (i < 10) {
do_thing_1;
do_thing_3;
} else {
do_thing_2;
}
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44886
Reviewed By: glaringlee
Differential Revision: D23768565
Pulled By: nickgg
fbshipit-source-id: 3fe40d91e82bdfff8dcb8c56a02a4fd579c070df
