Decode MMA kernel previously used scalar global→shared loads with
LD=HEAD_DIM+8 padding and per-tile scale multiply. This commit brings it
in line with the prefill MMA kernel (which already had these optimizations):
- cp.async K/V loads (bypasses registers, halves load instructions)
- XOR swizzle: LD=HEAD_DIM instead of HEAD_DIM+8 (zero waste smem)
- Pre-scale Q during load (removes per-tile scale multiply in softmax)
- Clean up prefill MMA kernel comments (no code change)
~2x speedup on decode (0.47ms→0.24ms at seq_len=512)
ViperEkura
e9b03f4fca