Can you provide direct link to the zipfile or instruction how to find it?
I can't find "IAR-EFR32xG12x-part-support-5.1.0" and "EFR32xG12x" in silabs search in any place except this answer.
What is "Software Releases"? Is it "Software downloads"? (http://www.silabs.com/products/development-tools/software)
If yes, there is no any IAR mention in this page.
IAR's latest release 8.11.2 has support for the EFR32xxx12 parts. You can create a project in simplicity studio for IAR and then open it in the IAR environment(with maybe a few tweaks). I have had no luck, however, attempting to run this code in the simplicity studio IDE, although it does seem to build correctly.
We work closely with IAR to ensure that the compiler supports our parts and our stacks. As a result, we can only guarantee that you will be able to build successfully and reliably with the versions specified.
Any version of IAR less than 8.11.2 (including your officially compatible version) does not support the EFR32xx12 parts. There seems to be no such thing as "IAR-EFR32xG12x-part-support-5.1.0" mentioned above. If I could find it, I would certainly try it on IAR 7.8 as that may be the best path. Please help us locate this essential set of files.