Optimize Silk encoder NEON paths and fix warped autocorrelation precision bug

[wangzihao3]

Summary

This PR applies targeted ARM NEON optimizations to two Silk encoder hot paths (~25% speedup, ~30% fewer instructions), and fixes a precision loss bug in the warped
autocorrelation computation.

Changes

silk/LPC_analysis_filter.c — New NEON implementation

• Add a NEON-vectorized silk_LPC_analysis_filter() that processes 8 filter taps per iteration using paired vmlal_s16 accumulators (int32x4_t).
• The original scalar implementation is preserved as silk_LPC_analysis_filter_c() for reference and OPUS_CHECK_ASM validation.

silk/arm/NSQ_del_dec_neon_intr.c — Multiple optimizations

• LPC prediction: Replace 16 individual vld1q_s32 loads with ldp (load-pair) inline assembly, loading 8 data vectors + 2 coefficient vectors per block. Use
  vqdmulhq_laneq_s32 to index directly into 128-bit coefficient vectors, eliminating vget_low/high_s32 split overhead.
• Allpass sections: Fully unroll the allpass loop via macro with compile-time branching on shapingLPCOrder, enabling precise register allocation and removing
  loop overhead.
• Common subexpression elimination: Precompute Tilt_Q14_Q16 and LF_shp_Q14_Q15 outside the per-sample loop; cache LF_AR_Q14 to avoid redundant vector
  loads.
• AR_shp_Q28 widening: Remove the scalar tail loop since MAX_SHAPE_LPC_ORDER (24) is exactly divisible by 8.
• State replacement copy: Unroll the numOthers copy loop 16× with __builtin_prefetch to hide memory latency during the candidate state swap.

silk/fixed/arm/warped_autocorrelation_FIX_neon_intr.c — Fix precision loss in autocorrelation

• The original code computed corr_QC[orderT] (the zero-lag autocorrelation) from the raw int16 input using vmull_s16, then left-shifted the result by QC.
  This lost precision because it operated on the un-warped source data instead of the warped signal that the rest of the function processes.
• The fix computes corr_QC[orderT] from the already-warped input_QS (int32) data using vmull_s32 + vsraq_n_s64 with the correct right shift 2*QS - QC,
  consistent with how all other corr_QC[] entries are accumulated. This ensures the autocorrelation is derived from the same warped signal path, eliminating the
  precision discrepancy.

Performance

Metric	Improvement
Encoding speed	~25% faster
Dynamic instruction count	~30% fewer