[InstCombine] Fold Select with binary op

Summary:
Fold
  %A = icmp eq i8 %x, 0
  %B = xor i8 %x, %z
  %C = select i1 %A, i8 %B, i8 %y
To
  %C = select i1 %A, i8 %z, i8 %y

Fixes https://bugs.llvm.org/show_bug.cgi?id=38345
Proof: https://rise4fun.com/Alive/43J

Reviewers: lebedev.ri, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49954

llvm-svn: 338300