[ARM][MC] Move information about variadic register defs into tablegen

[ARM][MC] Move information about variadic register defs into tablegen

Currently, variadic operands on an MCInst are assumed to be uses,
because they come after the defs. However, this is not always the case,
for example the Arm/Thumb LDM instructions write to a variable number of
registers.

This adds a property of instruction definitions which can be used to
mark variadic operands as defs. This only affects MCInst, because
MachineInstruction already tracks use/def per operand in each instance
of the instruction, so can already represent this.

This property can then be checked in MCInstrDesc, allowing us to remove
some special cases in ARMAsmParser::isITBlockTerminator.

Differential revision: https://reviews.llvm.org/D54853

llvm-svn: 348114