[llvm] [LoopFlatten] Enable by default (PR #214175)

[via llvm-commits]

github-actions[bot] wrote:

test-suite diff from dc14f0aecb1eb838e5cd83c220effc4f3dc1c848...564f11ab39202da7682797d9a72d9333252c7f04: https://github.com/llvm/llvm-project/actions/runs/32573584781/artifacts/9477195174

Statistics: https://github.com/llvm/llvm-project/actions/runs/32573584781/artifacts/9477195666

## riscv64-linux-gnu `-march=rva23u64 -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |             -0.000660387 |
## aarch64-linux-gnu `-march=armv9-a -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |              -0.00107903 |
## x86_64-linux-gnu `-march=x86-64-v3 -O3`
| ('Program', '')    |   ('size..text', 'base') |   ('size..text', 'head') |   ('size..text', 'diff') |
|:-------------------|-------------------------:|-------------------------:|-------------------------:|
| Geomean difference |                      nan |                      nan |             -0.000815772 |


https://github.com/llvm/llvm-project/pull/214175