[NFC][InstCombine] Add tests for sinking 'not' into 'xor' (PR38446)

https://rise4fun.com/Alive/IT3

Comes up in the [most ugliest]  signed int -> signed char  case of
-fsanitize=implicit-conversion (https://reviews.llvm.org/D50250)

Not sure if we want to do it always, or only when it is free to invert.

llvm-svn: 338967