Rename rsMaskVars (dotnet/coreclr#27012)

Rename rsMaskVars (dotnet/coreclr#27012)

* find src/jit -type f -exec sed -i -e 's/rsMaskVars->/GetMaskVars()->/g' {} \;

* Remove dangling comment

* Format patch

* Move to next line

* Update regset.h

* delete the comment after `public:`.

We have found them useless in the past and here it corrupts the formatting.

Commit migrated from https://github.com/dotnet/coreclr/commit/9d92b3b23309ebb5f65707e206a004d2115a6f91