Clean up sub-register implementation by moving subReg information back to

Clean up sub-register implementation by moving subReg information back to
MachineOperand auxInfo. Previous clunky implementation uses an external map
to track sub-register uses. That works because register allocator uses
a new virtual register for each spilled use. With interval splitting (coming
soon), we may have multiple uses of the same register some of which are
of using different sub-registers from others. It's too fragile to constantly
update the information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44104 91177308-0d34-0410-b5e6-96231b3b80d8