[X86] Remove code that unnecessarily sets EXTLOAD with src type of...

[X86] Remove code that unnecessarily sets EXTLOAD with src type of v2f32/v4f32/v8f32 as Legal for SSE2/AVX/AVX512 respectively. NFC

The LoadExt table defaults to all combinations being Legal. For
vector types, only src VTs with an i1 element type were ever changed.
So we don't need to mark them legal manually.

llvm-svn: 362170