Skip intermediate extractions

Skip intermediate extraction from a 2k accumulator register.

Examples: 1.

v32acc64  a = ...; // dmX
 
v16acc64 b = ext_v16acc64(0, a); // cmlX
v8acc64 c = ext_v8acc64(c, 1 b); // bmlhX

v32acc64  a = ...; // dmX
v8acc64 c = ext_v8acc64(1, a);