[VE] Support a basic disassembler for Aurora VE target

Summary:
Add a basic disassember and regression tests of LEA/LD/ST
instructions.  This patch also removes DecoderMethod declarations for
branch and call since those are not implemented in this patch.  They
will be added again later.  This patch also corrects DecoderMethod for
LD/ST instructions for one byte or two.

Differential Revision: https://reviews.llvm.org/D80912