intel/fs: Fix bug in lower_simd_width while splitting an instruction which was alread...

intel/fs: Fix bug in lower_simd_width while splitting an instruction which was already split.

This seems to be a problem in combination with the lower_regioning
pass introduced by a future commit, which can modify a SIMD-split
instruction causing its execution size to become illegal again.  A
subsequent call to lower_simd_width() would hit this bug on a future
platform.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
(cherry picked from commit bc781a0323d719634e29d82b5f14e22db943536e)