[clang] [llvm] [BPF] Add load-acquire and store-release instructions under -mcpu=v4 (PR #108636)

[Peilin Ye via cfe-commits]

peilin-ye wrote:

Back on this; rebased and pushed patchset v6 to make `acquire-release.ll` also cover `-march=bpfeb` as suggested by Eduard.  Basically:
```
; CHECK-LE: w0 = load_acquire((u8 *)(r1 + 0)) # encoding: [0xd3,0x10,0x00,0x00,0x10,0x00,0x00,0x00]
; CHECK-BE: w0 = load_acquire((u8 *)(r1 + 0)) # encoding: [0xd3,0x01,0x00,0x00,0x00,0x00,0x00,0x10]
```
`src_reg` and `dst_reg` are swapped, and `imm` is in big endian.  Currently focusing on making kernel-side changes (verifier and JITs).
- - -
That weird `fatal: reference is not a tree:` CI error looks similar to [the one mentioned in PR 73506](https://github.com/llvm/llvm-project/pull/73506#issuecomment-1832909676).  Running check-all locally for now...

https://github.com/llvm/llvm-project/pull/108636