Bug/performance: duplicate use of realpath()

[aristath]

While running Xdebug profiling with webgring in WP-Core, I noticed that realpath() is getting called too many times and costs a lot of time.
After some examination, I found that the register_block_style_handle() function calls realpath to set a $style_path_norm variable, and then a few lines below that, sets a $style_file variable with the exact same value, recalculating things and re-running realpath.

This is a simple bugfix and saves some performance. More details and screenshots in the PR

This patch removes duplicate calculations for a variable, reducing by half the number of times we call realpath().

Tests ran using Xdebug & webgind, attaching screenshots of the reports:

Before the patch:

After the patch:

Invocation count goes down from 639 to 461. Total self-cost goes down from 146ms to 89ms (values shown are in milliseconds).

Trac ticket: https://core.trac.wordpress.org/ticket/56636

[SergeyBiryukov]

In 54290:

Blocks: Remove duplicate use of realpath() in register_block_style_handle().

The register_block_style_handle() function called realpath() when retrieving the normalized style path, and then a few lines below that, recalculated the exact same value, running realpath() again.

This commit removes duplicate calculations, reducing the number of realpath() calls in the function by half. In tests ran using Xdebug & Webgrind, the total realpath() invocation count goes down from 639 to 461, and total self cost (the time that the function is responsible for) goes down from 146 ms to 89 ms.

Follow-up to [48141], [52291], [53091], [54155].

Props aristath.
Fixes #56636.

Thanks for the PR! Merged in r54290.