Cleveref's \crefrange and mixed equations and subequations

(Answer based on this post and @egreg's comment that the subequations environment management has changed.)

You can force the top-level label of the subequations environment to be treated as an equations-environment label through an optional argument to \label:

\begin{subequations}
\label[equation]{eqlabel}
[...]
\end{subequations}

Your MWE would look like this (extra sub-equation labels added to show this does not affect the top-level label behaviour):

\documentclass{article}

\usepackage{amsmath,cleveref}

\begin{document}

\begin{subequations}
\label[equation]{eqa}
\begin{align}
\label{subeqa1}
a &= b \\
\label{subeqa2}
c &= d
\end{align}
\end{subequations}

\begin{subequations}
\label[equation]{eqb}
\begin{align}
\label{subeqb1}
e &= f \\
\label{subeqb2}
g &= h
\end{align}
\end{subequations}

\begin{equation}
\label{eqc}
i = j
\end{equation}

Refer to \crefrange{eqa}{eqc}.

\end{document}

EDIT: The downside

It appears that labels declared as \label[equation]{...} are overlooked by RefTeX. This makes it a little less convenient to add references to top-level labels of a set of sub-equations if one is used to using RefTeX. I imagine there should be a simple way to fix this, but I do not know how.