This patch implements minimal VE code generation for empty function bodies (no args, no value return).
- empty function code generation test.
- Minimal function prologue & epilogue emission
- Instruction formats and instruction definitions as far as required for the empty function prologue & epilogue.
- I64 register class definitions.
This patch deviates from the common practice of having separate patches for the register file, instruction formats and instructions that offer self-contained logic but come without any tests. We are trying something new here: by implementing the (close to) minimum of everything in one patch, this patch is a minimal testable unit for VE code generation.