[X86] Lower v1i1/v2i1/v4i1/v8i1 load/stores to i8 load/store during op...

[X86] Lower v1i1/v2i1/v4i1/v8i1 load/stores to i8 load/store during op legalization if AVX512DQ is not supported.

We were previously doing this with isel patterns. Moving it to op legalization gives us chance to see the required bitcast earlier. And it lets us remove some isel patterns.

llvm-svn: 326669