Fix incorrect length for __builtin_bswap16.

The bswaphi pattern, corresponding to __builtin_bswap16, was missing the length attribute, causing the compiler to assume a default length of 8 bytes. On Microblaze, this could lead to the 8-byte bswaphi instruction being incorrectly scheduled into a branch instruction’s delay slot, which only accommodates 4-byte instructions. This patch corrects the length attribute, preventing this scheduling error.