RFR: 8241077: x86_64: Minor Assembler improvements

[Claes Redestad]

Hi,

a few small improvements extracted from 8241042 to keep that RFE
more focused:

- make InstructionAttr more compact by reordering some bool and int
fields leads to slightly better (and more compact) code generation.

- _is_managed only used on 32-bit

- code gen for *_needs_rex improve if replacing reg != (x)noreg
   with reg->is_valid(), since the expression compiles down into a
   range checks (fewer branches)

Bug:    https://bugs.openjdk.java.net/browse/JDK-8241077
Webrev: http://cr.openjdk.java.net/~redestad/8241077/open.00/

Testing: tier1-5 (together with JDK-8241042)

/Claes