I recently rebuilt my entire system using gcc 7.3, and from that moment on several Steam games crashed with a segmentation fault related to graphics/AMDGPU/LLVM (e.g. Black Mesa, Stellaris) while other games worked flawlessly (e.g. Civilization VI). After searching around I found the following bug report in mesa https://bugs.freedesktop.org/show_bug.cgi?id=101881 which lead to the following bug report in gcc https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81763. By creating my own gcc-7.3.0-r1 whose only difference from gcc-7.3.0 was the addition of the patch in https://gcc.gnu.org/viewcvs/gcc/branches/gcc-7-branch/gcc/config/i386/i386.md?view=patch&r1=257154&r2=257153&pathrev=257154 and rebuilding everything, the segmentation faults are gone. I would guess this could be of use for the wider Gentoo community while we wait for gcc 7.4 which will include the aforementioned patch.
The suggested patch seems identical to the patch from Mike Lothian to https://bugs.gentoo.org/show_bug.cgi?id=645776 (https://645776.bugs.gentoo.org/attachment.cgi?id=516792), so this is probably a duplicate bug.
*** This bug has been marked as a duplicate of bug 645776 ***
