Implement codegen for __builtin_choose_expr. For example:

struct X { int A; };

void foo() {
  struct X s;
  int i;
  i = __builtin_choose_expr(0, s, i);
}

compiles to:

        %tmp = load i32* %i             ; <i32> [#uses=1]
        store i32 %tmp, i32* %i

wow :)

llvm-svn: 40801