The function CPURegList::PopLowestIndex is called many times in the
arm64 simulator. However, the simulator does not need a full CPURegister
but only its register code. This CL creates a second PopLowestIndexAsCode
method which only returns the register code, and also marks the function
as inline. This speeds up the mjsunit/wasm/asm-wasm-f32 test from
1:45min to 1:35min in the optdebug build on my machine.

R=v8-arm-ports@googlegroups.com

Bug: v8:9396
Change-Id: I8cdcb2e0916dbb40e4a30ad5cd8f620b0358d08e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1803647
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63821}