Add the ability to compute exit values for complex loop using unanalyzable

operations.  This allows us to compile this testcase:

int main() {
        int h = 1;
         do h = 3 * h + 1; while (h <= 256);
        printf("%d\n", h);
        return 0;
}

into this:

int %main() {
entry:
        call void %__main( )
        %tmp.6 = call int (sbyte*, ...)* %printf( sbyte* getelementptr ([4 x sbyte]*  %.str_1, long 0, long 0), int 364 )        ; <int> [#uses=0]
        ret int 0
}

This testcase was taken directly from 256.bzip2, believe it or not.

This code is not as general as I would like.  Next up is to refactor it
a bit to handle more cases.

llvm-svn: 13019