i386: Integrate BFmode for Enhanced Vectorization in ix86_preferred_simd_mode

This change adds BFmode support to the ix86_preferred_simd_mode function
enhancing SIMD vectorization for BF16 operations. The update ensures
optimized usage of SIMD capabilities improving performance and aligning
vector sizes with processor capabilities.

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_preferred_simd_mode): Add BFmode Support.