[InstCombine][X86] If the lowest element of a scalar intrinsic isn't used make...

[InstCombine][X86] If the lowest element of a scalar intrinsic isn't used make sure we add it to the worklist so we can DCE it sooner.

We bypassed the intrinsic and returned the passthru operand, but we should also add the intrinsic to the worklist since its now dead. This can allow DCE to find it sooner and remove it. Similar was done for InsertElement when the inserted element isn't demanded.

llvm-svn: 290704