[DAGCombiner] don't hoist logic op if operands have other uses

The AVX512 diffs are neutral, but the bswap test shows a clear overreach in 
hoistLogicOpWithSameOpcodeHands(). If we don't check for other uses, we can 
increase the instruction count.

This could also fight with transforms trying to go in the opposite direction 
and possibly blow up/infinite loop. This might be enough to solve the bug 
noted here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181203/608593.html

I did not add the hasOneUse() checks to all opcodes because I see a perf 
regression for at least one opcode. We may decide that's irrelevant in the
face of potential compiler crashing, but I'll see if I can salvage that first.

llvm-svn: 348508