[profcheck] Add unknown branch weights to expanded cmpxchg loop. (#165841)

The AtomicExpandPass is responsible for lowering high-level atomic
operations (like `atomicrmw fadd`) that are unsupported by the target
hardware into a cmpxchg retry loop.

Given that we cannot empirically prove the precision branch weights, It
uses the `setExplicitlyUnknownBranchWeightsIfProfiled` function to
explicitly add "unknown" (50/50) branch weights to this branch.

This PR includes fies for the following tests:
```
Transforms/AtomicExpand/AArch64/atomicrmw-fp.ll
Transforms/AtomicExpand/AArch64/pcsections.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-f32-agent.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-f32-system.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-f64-agent.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-f64-system.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-rmw-nand.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-simplify-cfg-CAS-block.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-v2bf16-agent.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-v2bf16-system.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-v2f16-agent.ll
Transforms/AtomicExpand/AMDGPU/expand-atomic-v2f16-system.ll
Transforms/AtomicExpand/AMDGPU/expand-atomicrmw-fp-vector.ll
Transforms/AtomicExpand/ARM/atomicrmw-fp.ll
Transforms/AtomicExpand/LoongArch/atomicrmw-fp.ll
Transforms/AtomicExpand/Mips/atomicrmw-fp.ll
Transforms/AtomicExpand/PowerPC/atomicrmw-fp.ll
Transforms/AtomicExpand/RISCV/atomicrmw-fp.ll
Transforms/AtomicExpand/SPARC/libcalls.ll
Transforms/AtomicExpand/X86/expand-atomic-rmw-fp.ll
Transforms/AtomicExpand/X86/expand-atomic-rmw-initial-load.ll
Transforms/AtomicExpand/X86/expand-atomic-xchg-fp.ll
```

Co-authored-by: Jin Huang <jingold@google.com>

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