RFR: 8288300: AArch64: Remove the assertion in fmovs/fmovd(FloatRegister, FloatRegister) [v2]

[Hao Sun]

> The assertion, i.e. src and dst must be different registers, was
> introduced years ago. But I don't think it's needed.
> 
> This limitation was added in [1]. Frankly speaking, I don't know the
> reason. But I guess the assertion is probably used for debugging,
> raising one warning of fmovs/fmovd usage in the scenario of moving
> element at index zero from one **vector** register, to one float-point
> scalar register. If the "src" vector register and the "dst" float-point
> scalar register are the same one, it introduces a side-effect, i.e. the
> higher bits are cleared to zeros[2].
> 
> If so, I argue that
> 1) the assembler should align with the ISA.
> 2) compiler developers should be aware of the side-effect when they want
> to use fmovs/fmovd, and they should guarantee "dst != src" if they like
> to higher bits untouched, e.g., [3].
> 
> Hence, I think we can remove this unnecessary assertion.
> 
> [1] http://hg.openjdk.java.net/aarch64-port/jdk8/hotspot/rev/9baee4e65ac5
> [2] https://developer.arm.com/documentation/ddi0596/2021-12/SIMD-FP-Instructions/FMOV--register---Floating-point-Move-register-without-conversion-?lang=en
> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64_sve.ad#L4899

Hao Sun has updated the pull request incrementally with one additional commit since the last revision:

  Remove unnecessary whitespace
  
  As suggested by aph, remove the unnecessary whitespace and keep the
  substantive changes only.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9163/files
  - new: https://git.openjdk.org/jdk/pull/9163/files/58361d8b..4949c37e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9163&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9163&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/9163.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9163/head:pull/9163

PR: https://git.openjdk.org/jdk/pull/9163