ARM scheduling fix: don't guess at implicit operand latency.

This is a minor drive-by fix with no robust way to unit test.
As an example see neon-div.ll:
SU(16):   %Q8<def> = VMOVLsv4i32 %D17, pred:14, pred:%noreg, %Q8<imp-use,kill>
 val SU(1): Latency=2 Reg=%Q8
...should be latency=1

llvm-svn: 158960