Rollup merge of #151958 - chahar-ritik:add-slp-vectorization-test, r=jieyouxu

Add codegen test for SLP vectorization

close: rust-lang/rust#142519

This PR adds a codegen regression test for rust-lang/rust#142519. A regression in LLVM to fail to auto-vectorize, leading to significant performance loss.

The SLP vectorizer correctly groups the 4-byte operations into <4 x i8> vectors.

The loop state is maintained in SIMD registers (phi <4 x i8>).

The test remains robust across architectures (AArch64 vs x86_64) by allowing flexible store types (i32 or <4 x i8>).

Author: jhpratt