llama : make tensor-split regex patterns static (#24710)

llama_meta_device_get_split_state() recompiled 29 std::regex on every call. In -sm tensor mode the callback runs once per tensor per token, so this dominated the decode thread in profiling. Mark them static const so they are compiled once. Kept inside the function (local statics are thread-safe since C++11). Patterns are literal and stateless, so behavior is unchanged.