Apparently, Lenovo released a new BIOS update for my laptop (Thinkpad E485)
today. The changelog mentions, "Sync IOAPICID in IVRS and APIC ACPI tables
(Linux)." I installed the update and removed both the ivrs_ioapic[32]=00:14.0
flag, as well as the iommu=pt flag from my kernel command line for testing. To
my surprise, the laptop booted just fine and from what I can tell, the
graphical corruption seems to be gone. In the meantime, the kernel and mesa
were updated a few times and I cannot pinpoint what, exactly, fixed my issues.

-> mesa 19.1.5
-> kernel 5.2.10.arch1-1
-> llvm-libs 8.0.1-3