[X86] Add test cases to show bad type legalization of fptosi/fptosui...

[X86] Add test cases to show bad type legalization of fptosi/fptosui v16f32->v16i8 and v8f64->v8i16 on pre-AVX512 targets.

When splitting the v16f32/v8f64 result type, type legalization will try to promote the integer result type before a concat and an explicit truncate. But for the fptoui test case this is particularly bad since fptoui isn't supported on X86 until AVX512. We could use an fptosi since the result range would fit in a signed 32-bit value, but the generic type legalization doesn't do that transformation when splitting. It does do this when promoting.

llvm-svn: 347533