Adding a new answer, as even if you get install to work - boot seems very unreliable. I have tried the very latest Ubuntu 19.04 pre-release (2019-02-20 updates) and Ubuntu 18.10 - and it consistently freezes / lock-up during boot.
To get install and boot to work at all, common advice for this Dell 7375 is to start with:
amd_iommu=on ivrs_ioapic[4]=00:14.0 ivrs_ioapic[5]=00:00.2
I tried hundreds of boots, and almost always from a cold boot (at least 10 seconds off to let RAM drain). No matter what, I would be lucky to get 1 boot out of 3 all the way past login. Sometimes the graphics would start very slow and cursor would appear in bottom right, but the login screen not draw, but more often than not it would hang before even getting into graphics / login prompt.
Also be sure to grep your dmesg and check your BIOS level. In all these Ryzen Vega Linux issues, from any vendor of laptop, BIOS is cited as the main cause for all these problems. So make sure we are all starting with the most recent. The latest avaialble I found was (from my dmesg):
Dell Inc. Inspiron 7375/0GTNWX, BIOS 1.5.0 07/09/2018
I suggest you remove both splash and quiet from GRUB so you can see where things are hanging. And what I noticed is that every time it worked, the framebuffer driver would change about 1/3 through the console messages (dmesg). But the failed attempts did not clear the screen during boots.
This proved to be a key clue, as I found a user running Slackware 14.2 who focused on that console boot issue on this exact model laptop. I ended up with less-mentioned information here: https://www.linuxquestions.org/questions/slackware-14/amdgpu-raven-screen-freeze-on-switch-from-efifb-to-amdgpudrmfb-4175636284/
Which focused on the console frame buffer driver (amdgpudrmfb) only working some of the time. Then I changed my /etc/default/grub to have:
GRUB_CMDLINE_LINUX_DEFAULT="amdiommu=on ivrs_ioapic[4]=00:14.0 ivrs_ioapic[5]=00:00.2 acpi_backlight=vendor nomodeset vga=normal"
And now my system boots every single time!