[PatternMatch] Add m_SExtOrSelf(), m_ZExtOrSExtOrSelf() matchers + unittests

m_SExtOrSelf() is for consistency.

m_ZExtOrSExtOrSelf() is motivated by the D68103/r373106 :
sometimes it is useful to look past any extensions of the shift amount,
and m_ZExtOrSExtOrSelf() may be exactly the tool to do that.

llvm-svn: 373128