[InstCombine] add folds for icmp with xor mask constant

[InstCombine] add folds for icmp with xor mask constant

These are the folds in Alive;
Name: xor_ult
Pre: isPowerOf2(-C1)
%xor = xor i8 %x, C1
%r = icmp ult i8 %xor, C1
=>
%r = icmp ugt i8 %x, ~C1

Name: xor_ugt
Pre: isPowerOf2(C1+1)
%xor = xor i8 %x, C1
%r = icmp ugt i8 %xor, C1
=>
%r = icmp ugt i8 %x, C1

https://rise4fun.com/Alive/Vty

The ugt case in its simplest form was already handled by DemandedBits,
but that's not ideal as shown in the multi-use test.

I'm not sure if these are all of the symmetrical folds, but I adjusted
the existing code for one of the folds to try to show the similarities.

There's no obvious connection, but this is another preliminary step
for PR14613...
https://bugs.llvm.org/show_bug.cgi?id=14613

llvm-svn: 341997