cmd/compile/internal/types2: fix conversions of constants to type parameter
When converting a constant to a type parameter, the result is never
constant (type parameters are not constant types), but we still need
to verfy that the constant is representable by each specific type in
the type set of the type parameter.
Fixes #49247.
Change-Id: I460983c7070b33baadce25dd23210e10930cfb2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/360396
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>