[InstSimplify] fold div/rem of zexted bool

I was looking at an unrelated fold and noticed that
we don't have this simplification (because the other
fold would break existing tests).

Name: zext udiv
  %z = zext i1 %x to i32
  %r = udiv i32 %y, %z
=>
  %r = %y

Name: zext urem
  %z = zext i1 %x to i32
  %r = urem i32 %y, %z
=>
  %r = 0

Name: zext sdiv
  %z = zext i1 %x to i32
  %r = sdiv i32 %y, %z
=>
  %r = %y

Name: zext srem
  %z = zext i1 %x to i32
  %r = srem i32 %y, %z
=>
  %r = 0

https://rise4fun.com/Alive/LZ9

llvm-svn: 335512