As Walter mentioned, it is not possible to load a 32-bit shared library (DLL) into a 64-bit version of MATLAB using the loadlibrary command.
MATLAB R2015b was the last MATLAB release which had a 32-bit version. Therefore, you should install a 32-bit MATLAB which is R2015b or older.
After doing this, you might not need to install a new 32-bit compatible compiler since 32-bit MATLAB versions usually have the 'lcc-win32' compiler installed by default. See the 'supported compilers' page corresponding to your MATLAB release for more information about this:
Please note that loadlibrary has a few different limitations for importing C DLLs into MATLAB:
The above link can help verify if your header file and DLL is compatible with MATLAB's loadlibrary function.