Handle sub-register operands in recomputeRegClass().

Handle sub-register operands in recomputeRegClass().

Now that getMatchingSuperRegClass() returns accurate results, it can be
used to compute constraints imposed by instructions using a sub-register
of a virtual register.

This means we can recompute the register class of any virtual register
by combining the constraints from all its uses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146874 91177308-0d34-0410-b5e6-96231b3b80d8