[Bug gas/22871] Encode instructions of 64-bit registers without the REX_W bit

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=22871

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jan Beulich from comment #1)
> subq %r64, %r64 and testq $imm31, %r64 could similarly benefit. Whether

I added subq and testq to users/hjl/optimize branch at

https://github.com/hjl-tools/binutils-gdb/tree/users/hjl/optimize

> converting test{q,l,w} $imm8,%r{64,32,16} to testb $imm8,%r8 isn't as clear,
> but I think reads of 8-bit registers are commonly not causing stalls (only
> writes do).
> 
> Along those lines test{q,l,w} $imm,mem might allow conversion if all set
> bits in imm fall within a single byte. Of course care needs to be taken that
> the adjustment to the displacement won't break addressability of the object.

Let's go with 64-bit register cases first and see how they go.

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