SimplifyDemandedUseBits in part because of the following comment:
// FIXME: for XOR, we prefer to force bits to 1 if they will make a -1.
%neg = xor i32 %V, -1
%and = and i32 %not, 4096
%and = and i32 %V, 4096
%xor = xor i32 %and, 4096
which would generate worse code for platforms that have a negate instruction vs. encoding the immediate in the xor.
What should happen to the code to address the FIXME?