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For hung off uses, we need a Use* to tell use where the operands are. This was User::OperandList but we want to remove that to save space of all subclasses which aren't making use of 'hung off uses'. Hung off uses now allocate their own 'OperandList' Use* in the User::new which they call. getOperandList() now uses the hung off uses bit to work out where the Use* for the OperandList lives. If a User has hung off uses, then this bit tells them to go back a single Use* from the User* and use that value as the OperandList. If a User has no hung off uses, then we get the first operand by subtracting (NumOperands * sizeof(Use)) from the User this pointer. This saves a pointer from User and all subclasses. Given the average size of a subclass of User is 112 or 128 bytes, this saves around 7% of space With malloc tending to align to 16-bytes the real saving is typically more like 3.5%. On 'opt -O2 verify-uselistorder.lto.bc', peak memory usage prior to this change is 149MB and after is 143MB so the savings are around 2.5% of peak. Looking at some passes which allocate many Instructions and Values, parseIR drops from 54.25MB to 52.21MB while the Inliner calls to Instruction::clone() drops from 28.20MB to 27.05MB. Reviewed by Duncan Exon Smith. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239623 91177308-0d34-0410-b5e6-96231b3b80d8