backend/x64: Utilize extended EVEX registers as Spill-Space

[Wunkolo]

AVX512 adds an additional 16 {x,y,z}mm registers to the register-state. We only address the xmm part of these registers, so this allows an additional 256-bytes of fast register-memory. These registers cannot be utilized immediately without updating all of our instructions to conditionally utilize EVEX encoding to be able to address these additional registers. Instead, these additional registers can be utilized as fast additional spill-space before having to utilize system memory.