[VE] Support branch instructions in MC layer

Summary:
Add regression tests of asmparser, mccodeemitter, and disassembler for
branch instructions.  In order to support them, we enhance asmparser
by adding splitting mnemonic mechanism, e.g. "bgt.l.t" into "b", "gt",
and ".l.t", and parsing mechanism for AS style memory addressing.
We also implment encoding and decoding mechanism for branch instructions.

Differential Revision: https://reviews.llvm.org/D81215