x86: re-work AVX512 embedded rounding / SAE

As a preparatory step to allowing proper non-operand forms of specifying
embedded rounding / SAE, convert the internal representation to non-
operand form. While retaining properties (and in a few cases perhaps
providing more meaningful diagnostics), this means doing away with a few
hundred standalone templates, thus - as a nice side effect - reducing
memory consumption / cache occupancy.