The arm64 backend defines registers with a common base `CPURegister` class which
can represent both general purpose and vector registers. We would use it to
define the `RegisterName` function which results in printing all registers with
`xN` when using the --trace-turbo-graph flag:

~~~
[x0|R|f64] = Arm64LdrD : MRR [x7|R|tp] [x5|R|w64]
 ^^
 This is the d0 register, not x0
~~~

We have `Register` and `VRegister` classes to distinguish general purpose
registers from vector registers, use those to define `RegisterName` functions
and print vector registers as `vN` intead:

~~~
[v0|R|f64] = Arm64LdrD : MRR [x7|R|tp] [x5|R|w64]
~~~

Since FloatRegister, DoubleRegister and Simd128Register are typedef of
VRegister, we cannot differentiate them with the current `DEFINE_REGISTER_NAMES`
abstraction. Architecturaly, S, D and Q registers are aliases of V registers so
that's not a problem.

Change-Id: Ic43036117c834070d3311b65c99ad1e24e1f9c3f
Reviewed-on: https://chromium-review.googlesource.com/c/1445990Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#59234}