Teach the DAGISelEmitter to not compute the variable_ops operand

index for the input pattern in terms of the output pattern. Instead
keep track of how many fixed operands the input pattern actually
has, and have the input matching code pass the output-emitting
function that index value. This simplifies the code, disentangles
variables_ops from the support for predication operations, and
makes variable_ops more robust.

llvm-svn: 51808