Generated IROp_Header is not properly aligned
In #2319 hasDest is removed from struct IROp_Header, which makes IROp_Header 3 bytes in total. Since all IR structs are packed, and referenced IROp_Header, which makes them all un-aligned. This could: 1. have performance impact on IR emitting. 2. breaks some compilers like clang in NDK (which addes 1 byte padding in derived IROp_XXX structures).