[MCA][InstrBuilder] Check for the presence of flag VariadicOpsAreDefs.

[MCA][InstrBuilder] Check for the presence of flag VariadicOpsAreDefs.

This patch fixes the logic that checks for variadic register definitions,

Before llvm-svn 348114 (commit 4cf35b4ab0b), it was not possible to explicitly
mark variadic operands as definitions. By default, variadic operands of an
MCInst were always assumed to be uses. A number of had-hoc checks were
introduced in the InstrBuilder to fix the processing of variadic register
operands of ARM ldm/stm variants.

This patch simply replaces those old (and buggy) checks with a much simpler (and
correct) check for MCID::Flag::VariadicOpsAreDefs.