[InstCombine] When adding an Instruction and its Users to the worklist at the...

[InstCombine] When adding an Instruction and its Users to the worklist at the same time, make sure we put the Users in first. Then put in the instruction.

This way we ensure we immediately revisit the instruction and do any additional optimizations before visiting the users. Otherwise we might visit the users, then the instruction, then users again, then instruction again.

llvm-svn: 299267