[X86][AVX] Added support for lowering to VBROADCASTF128/VBROADCASTI128 with generic IR

As discussed on D22460, I've updated the vbroadcastf128 pd256/ps256 builtins to map directly to generic IR - load+splat a 128-bit vector to both lanes of a 256-bit vector.

Fix for PR28657.

llvm-svn: 276417