radeonsi: merge 2 conditional blocks with same condition into 1 in culling code

radeonsi: merge 2 conditional blocks with same condition into 1 in culling code

The block only loads input VGPRs from LDS, and the next block uses them.
The entering condition is the same, even though the second block is
the next shader part beginning with the prolog.

Simply move the VGPR loads into the prolog.

This decreases the shader code size by 12 bytes.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11102>