Fix operand latency computation in cases where the definition operand is

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116080 91177308-0d34-0410-b5e6-96231b3b80d8