[InstCombine] Change the interface of SimplifyDemandedBits so that it takes...

[InstCombine] Change the interface of SimplifyDemandedBits so that it takes the instruction and operand instead of the Use.

The first thing it did was get the User for the Use to get the instruction back. This requires looking through the Uses for the User using the waymarking walk. That's pretty fast, but its probably still better to just pass the Instruction we already had.

llvm-svn: 298772