Fix the SD scheduler to avoid gluing the same node twice.

DAGCombine strangeness may result in multiple loads from the same
offset. They both may try to glue themselves to another load. We could
insist that the redundant loads glue themselves to each other, but the
beter fix is to bail out from bad gluing at the time we detect it.

Fixes rdar://11314175: BuildSchedUnits assert.

llvm-svn: 155668