It can definitely help in certain domains, but adding special-case instructions in silicon can sometimes complicate a chip design enough that it slows it down overall. The trend for a while was in the other direction, towards not implementing in silicon things that were even already in the x86 instruction set, like the transcendental arithmetic functions, and doing them in microcode instead (the "RISCification" of x86 processors). It's possible that trend is now reversing, though.