Maybe this is the complement of bug 1158704: I don't know why ucode-intel was
installed on my PC with an AMD Phenom II CPU, but when updating it, the initrd
is being rebuilt.
That does not make sense for the two obvious reasons:
1) The AMD CPU cannot use Intel microcode
2) There is no need to include Intel Microcode in the initrd of an AMD machine

Looking further the details are more amazing:
# lsinitrd 
Image: /boot/initrd-4.12.14-lp151.28.52-default: 20M
========================================================================
Early CPIO image
========================================================================
drwxr-xr-x   1 root     root            0 Jun 26 12:49 .
-rw-r--r--   1 root     root            2 Jun 26 12:49 early_cpio
drwxr-xr-x   1 root     root            0 Jun 26 12:49 kernel
drwxr-xr-x   1 root     root            0 Jun 26 12:49 kernel/x86
drwxr-xr-x   1 root     root            0 Jun 26 12:49 kernel/x86/microcode
-rw-r--r--   1 root     root        12684 Jun 26 12:49
kernel/x86/microcode/AuthenticAMD.bin
========================================================================
Version: dracut-044-lp151.2.18.1

Arguments: --logfile --force
[...]

So the Intel microcode is not present in the initrd image. So why then update
it? The posttrans script does not do any CPU test.

The CPU I have is a "AMD Phenom(tm) II X4 840 Processor".