make some fairly meaty internal changes to how SimplifyDemandedBits works.

make some fairly meaty internal changes to how SimplifyDemandedBits works.
Now, if it detects that "V" is the same as some other value,
SimplifyDemandedBits returns the new value instead of RAUW'ing it immediately.
This has two benefits:
1) simpler code in the recursive SimplifyDemandedBits routine.
2) it allows future fun stuff in instcombine where an operation has multiple
   uses and can be simplified in one context, but not all.

#2 isn't implemented yet, this patch should have no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63479 91177308-0d34-0410-b5e6-96231b3b80d8