Instcombine: cast (X != 0) to int, cast (X == 1) to int -> X iff X has only the low bit set.

This implements set.ll:test20.

This triggers 2x on povray, 9x on mesa, 11x on gcc, 2x on crafty, 1x on eon,
6x on perlbmk and 11x on m88ksim.

It allows us to compile these two functions into the same code:

struct s { unsigned int bit : 1; };
unsigned foo(struct s *p) {
  if (p->bit)
    return 1;
  else
    return 0;
}
unsigned bar(struct s *p) { return p->bit; }

llvm-svn: 21690