RFR: 8241042: x86_64: Improve Assembler generation

[Claes Redestad]

Hi,

on x64, folding consecutive bytes emitted by Assembler routines into
byte-wise variants of emit_int16, emit_int24 and emit_32 turns out to
be profitable, since gcc can't seem to fold the back to back stores
to the end() pointer into one. The new methods are also a bit
convenient.

Together with a few other small fixes, this adds up to around a 0.4%
reduction in instructions retired on Hello World.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8241042
Webrev: http://cr.openjdk.java.net/~redestad/8241042/open.00/

Testing: tier1-5 (earlier version with no semantic differences), tier1-2
(current patch)

Thanks!

/Claes