Only align after assignments on the top level.

This fixes llvm.org/PR14870 and we no longer mess up:
template <typename T1, typename T2 = char, typename T3 = char,
          typename T4 = char>
void f();

It removes the nice aligment for assignments inside other expressions,
but I am not sure those are actually practically relevant. If so, we can
fix those later.

llvm-svn: 171966