[X86] Always emit an integer vbroadcast_load from lowerBuildVectorAsBroadcast...

[X86] Always emit an integer vbroadcast_load from lowerBuildVectorAsBroadcast regardless of AVX vs AVX2

If we go with D75412, we no longer depend on the scalar type directly. So we don't need to avoid using i64. We already have AVX1 fallback patterns with i32 and i64 scalar types so we don't need to avoid using integer types on AVX1.

Differential Revision: https://reviews.llvm.org/D75413