RFR: 8241095: x86: Improve prefix handling in Assembler

[Claes Redestad]

Hej,

this patch enable folding emitted prefixq bytes into multi-byte emitter
functions (emit_int16(int8, int8) etc).

Only those functions where there is always a prefix being emitted have
been adjusted. Other variants could improve slightly by doing similar
tricks, but with a smaller payoff and more complexity.

Webrev: http://cr.openjdk.java.net/~redestad/8241095/open.00
Bug:    https://bugs.openjdk.java.net/browse/JDK-8241095

Testing: tier1-4 (ongoing)

Thanks,

/Claes