Fix operand latency computation in cases where the definition operand is

implicit. e.g.
%D6<def>, %D7<def> = VLD1q16 %R2<kill>, 0, ..., %Q3<imp-def>                                                                                                                                                                              
%Q1<def> = VMULv8i16 %Q1<kill>, %Q3<kill>, ...

The real definition indices are 0,1.

llvm-svn: 116080