[Hexagon] Fix the value of HexagonII::TypeCVI_FIRST

This value is the first vector instruction type in numerical order. The
previous value was incorrect, leaving TypeCVI_GATHER outside of the range
for vector instructions. This caused vector .new instructions to be
incorrectly encoded in the presence of gather.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335065 91177308-0d34-0410-b5e6-96231b3b80d8