[InstCombine] fold sub of low-bit masked value from offset of same value

[InstCombine] fold sub of low-bit masked value from offset of same value

There might be some demanded/known bits way to generalize this,
but I'm not seeing it right now.

This came up as a regression when I was looking at a different
demanded bits improvement.

https://rise4fun.com/Alive/5fl

  Name: general
  Pre: ((-1 << countTrailingZeros(C1)) & C2) == 0
  %a1 = add i8 %x, C1
  %a2 = and i8 %x, C2
  %r = sub i8 %a1, %a2
  =>
  %r = and i8 %a1, ~C2

  Name: test 1
  %a1 = add i8 %x, 192
  %a2 = and i8 %x, 10
  %r = sub i8 %a1, %a2
  =>
  %r = and i8 %a1, -11

  Name: test 2
  %a1 = add i8 %x, -108
  %a2 = and i8 %x, 3
  %r = sub i8 %a1, %a2
  =>
  %r = and i8 %a1, -4