x86: simplify SAE checking

To determine whether SAE (with or without StaticRounding) is permitted
there's no need to iterate over all operands. Even less so starting at
the front (thus needlessly inspecting immediate operands as well).
Leverage the pattern across all relevant templates and check only the
last two operands, and also only for non-512 ones (besides the non-LIG
case that was already checked for).