[NNC] Horizontally fuse all loops (#56324)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56324
Inlining is great if LLVM's CSE kicks in; but if a kernel has multiple outputs
(and thus multiple loops), CSE has no chance.
So, this pass "horizontally" fuses the output loops together so that CSE can go
to town. Essentially we want to turn
```
for (...) {
output_1[] = some_complicated_expr...
}
for (...) {
output_2[] = some_complicated_expr...
}
```
Into:
```
for (...) {
output_1[] = complicated_expr
output_2[] = complicated_expr. // llvm cse should take care of this
}
```
Test Plan: Imported from OSS
Reviewed By: navahgar
Differential Revision: D27841194
Pulled By: bertmaher
fbshipit-source-id: 54153bb59786be87183c636d64f05963c4b1624a
