Auto merge of #108012 - compiler-errors:issue-107999, r=oli-obk

Don't ICE in `might_permit_raw_init` if reference is polymorphic

Emitting optimized MIR for a polymorphic function may require computing layout of a type that isn't (yet) known. This happens in the instcombine pass, for example. Let's fail gracefully in that condition.

cc `@saethlin`
fixes #107999

Author: bors