Rollup merge of #123635 - maurer:kcfi-no-assoc, r=compiler-errors

CFI: Fix ICE in KCFI non-associated function pointers

We oddly weren't testing the more usual case of casting non-methods to function pointers. The KCFI shim insertion logic would ICE on these due to asking for an irrefutable associated item if we cast a function to a function pointer without needing a traditional shim.

r? `@compiler-errors`