Add a new kind of MachineOperand: MO_RegisterMask.

Register masks will be used as a compact representation of large clobber
lists.  Currently, an x86 call instruction has some 40 operands
representing call-clobbered registers.  That's more than 1kB of useless
operands per call site.

A register mask operand references a bit mask of call-preserved
registers, everything else is clobbered.  The bit mask will typically
come from TargetRegisterInfo::getCallPreservedMask().

By abandoning ImplicitDefs for call-clobbered registers, it also becomes
possible to share call instruction descriptions between calling
conventions, and we can get rid of the WINCALL* instructions.

This patch introduces the new operand kind.  Future patches will add
RegMask support to target-independent passes before finally the fixed
clobber lists can be removed from call instruction descriptions.

llvm-svn: 148250