Replace all accesses to User::OperandList with getter and setter methods. NFC.

Replace all accesses to User::OperandList with getter and setter methods. NFC.

We don't want anyone to access OperandList directly as its going to be removed
and computed instead.  This uses getter's and setter's instead in which we
can later change the underlying implementation of OperandList.

Reviewed by Duncan Exon Smith.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239620 91177308-0d34-0410-b5e6-96231b3b80d8