[AArch64][test] Add i256 codegen baseline tests (#183587)
## Summary
Add baseline codegen tests for i256 operations on AArch64:
- `bitcnt-i256.ll`: ctpop, ctlz, cttz, Hamming distance, parity (3 RUN
lines: +neon, +sve, +cssc)
- `cmp-i256.ll`: 8 icmp variants (slt/sgt/sle/sge/ult/ugt/ule/uge) +
select
- `div-i256.ll`: udiv, sdiv, urem, srem, power-of-2, and constant
division
- `mul-i256.ll`: multiply, multiply-by-constant, add, sub, and, or, xor
- `shift-i256.ll`: variable shl/lshr/ashr, constant shifts, narrow (i32)
amounts
These are test-only NFC pre-patches for `__int256` builtin type support
(#182733). i256 is already a valid LLVM IR type; these tests capture the
current baseline codegen. The main PR will update the CHECK lines to
show codegen improvements (e.g., direct libcall routing for
division/multiply).
## Test plan
- All 5 tests pass with `llvm-lit` on upstream `main` (generated with
`update_llc_test_checks.py` using upstream `llc`)
## AI Disclosure
This patch was developed with assistance from Claude (Anthropic). All
commits carry `Assisted-by: Claude (Anthropic)` and `Co-Authored-By:
Claude Opus 4.6 <noreply@anthropic.com>` trailers, per the [LLVM AI Tool
Policy](https://llvm.org/docs/DeveloperPolicy.html#use-of-ai-generated-contributions).
All code has been reviewed and validated by the author.
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
