Remove variable_ops from call instructions in most targets.

Call instructions are no longer required to be variadic, and
variable_ops should only be used for instructions that encode a variable
number of arguments, like the ARM stm/ldm instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160189 91177308-0d34-0410-b5e6-96231b3b80d8