[NNC] Add simplification of Loop + Condition patterns. (#44764)
Summary:
Adds a new optimization to the IRSimplifier which changes this pattern:
```
for ...
if ...
do thing;
```
into:
```
if ...
for ...
do thing;
```
Which should be almost strictly better.
There are many cases where this isn't safe to do, hence tests. Most obviously when the condition depends on something modified within the loop.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44764
Reviewed By: mruberry
Differential Revision: D23734463
Pulled By: nickgg
fbshipit-source-id: 51617e837de96b354fb702d0090ac65ddc523d36
