[LoongArch] Add tests showing the optimization pipeline

Other targets like ARM, AArch64, RISCV and X86 have similar tests.

`O1`, `O2` and `O3` appear to be the same for now. But in future, some
passes may be disabled at lower levels (e.g. `O1`). Hoping we can use
FileCheck prefixes for differences to avoid repeating the contents 3
times.

Reviewed By: xen0n, MaskRay

Differential Revision: https://reviews.llvm.org/D139499