`NumRegs` runs a `population count` and must be used with
a `RegList` and not with a regular integer value.

kCallerSavedDoubles is a regular integer and should be used as is.

Change-Id: Id9535134ad4ea02bebed9b506012084d93acc2c2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2965159Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#75213}