compiler: in composite literals use temps only for interfaces

For a composite literal we only need to introduce a temporary variable
if we may be converting to an interface type, so only do it then.
This saves over 80% of compilation time when using gccgo to compile
cmd/internal/obj/x86, as the GCC middle-end spends a lot of time
pointlessly computing interactions between temporary variables.

For PR debug/101064
For golang/go#46600

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/331513

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