Add support for partial redefs to the fast register allocator.

Add support for partial redefs to the fast register allocator.

A partial redef now triggers a reload if required. Also don't add
<imp-def,dead> operands for physical superregisters.

Kill flags are still treated as full register kills, and <imp-use,kill> operands
are added for physical superregisters as before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104167 91177308-0d34-0410-b5e6-96231b3b80d8