[DAGCombiner] after simplifying demanded elements of vector operand of...

[DAGCombiner] after simplifying demanded elements of vector operand of extract, revisit the extract; 2nd try

This is a retry of rL349051 (reverted at rL349056). I changed the check for dead-ness from
number of uses to an opcode test for DELETED_NODE based on existing similar code.

Differential Revision: https://reviews.llvm.org/D55655

llvm-svn: 349058