intel/compiler: Nest definition of live variables block_data structures

intel/compiler: Nest definition of live variables block_data structures

When this commit was originally written, these two structures had the
exact same name. Subsequently in commit 12a8f2616a2f (intel/compiler:
Fix C++ one definition rule violations) they were renamed.

Original commit message:

> These two structures have exactly the same name which prevents the two
> files from being included at the same time and could cause serious
> trouble in the future if it ever leads to a (silent) violation of the
> C++ one definition rule.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4012>