[VE] fastcc and vreg-to-vreg copy

This defines a 'fastcc' for the VE target and implements vreg-to-vreg
copy for parameter passing.  The 'fastcc' extends the standard CC for
SX-Aurora with register passing of vector-typed parameters and return
values.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D90842