Fix an assertion in DwarfExpression when emitting fragments in vector registers

When DwarfExpression is emitting a fragment that is located in a
register and that fragment is smaller than the register, and the
register must be composed from sub-registers (are you still with me?)
the last DW_OP_piece operation must not be larger than the size of the
fragment itself, since the last piece of the fragment could be smaller
than the last subregister that is being emitted.

rdar://problem/29779065

llvm-svn: 290324