Use a latency value of 0 for the artificial edges inserted by

Use a latency value of 0 for the artificial edges inserted by
AddPseudoTwoAddrDeps. This lets the scheduling infrastructure
avoid recalculating node heights. In very large testcases this
was a major bottleneck. Thanks to Roman Levenstein for finding
this!

As a side effect, fold-pcmpeqd-0.ll is now scheduled better
and it no longer requires spilling on x86-32.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61778 91177308-0d34-0410-b5e6-96231b3b80d8