[clang] [CIR][AArch64] Upstream vrshrd_n_s64/u64 and vrshr_n_v vector rounding shift right (PR #185992)

[Md Mouzam Arfi Hussain via cfe-commits]

ArfiH wrote:

Thank you for the continued feedback. All comments have been addressed:

- Removed `bool isUnsigned` in `vrshr_n_v` — now uses `usgn` directly
- Restored removed vrshr blocks in `neon-intrinsics.c` with `CHECK-SAME` lines
- Added missing EOF newline

https://github.com/llvm/llvm-project/pull/185992