[VE] Minimal codegen for empty functions

Summary:
This patch implements minimal VE code generation for empty function bodies (no args, no value return).

Contents

* 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.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D72598