Fixes#3400. The frontend pre-casts the static array operand to the
slice type for cases like `bool[4] == bool[]`, but that's not possible
in case the static array is non-mutable but the slice type is mutable.
So check the element types for equivalence, not the array types, as
static arrays and slices are NOT equivalent.
This also fixes a pessimization where the `memcmp` call would become an `invoke` if the user provides his own `memcmp` prototype in the code (the prototype would not carry the `nounwind` function attribute).
