Fix a bug handling nodes with variable arguments. The code was fixed to assume

Fix a bug handling nodes with variable arguments.  The code was fixed to assume
that there were two input operands before the variable operand portion.  This
*happened* to be true for all call instructions, which took a chain and a
destination, but was not true for the PPC BCTRL instruction, whose destination
is implicit.

Making this code more general allows elimination of the custom selection logic
for BCTRL.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31732 91177308-0d34-0410-b5e6-96231b3b80d8