Move OperandList to be allocated prior to User for hung off subclasses.

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.

llvm-svn: 239623