Change the REG_SEQUENCE SDNode to take an explict register class ID as its first...

Change the REG_SEQUENCE SDNode to take an explict register class ID as its first operand.  This operand is lowered away by the time we reach MachineInstrs, so the actual register-allocation handling of them doesn't need to change.
This is intended to support using REG_SEQUENCE SDNode's with type MVT::untyped, and is part of the long road to eliminating some of the hacks we currently use to support register pairs and other strange constraints, particularly on ARM NEON.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133178 91177308-0d34-0410-b5e6-96231b3b80d8