[X86] Simplify the end of custom type legalization for (v2i32/v4i16/v8i8...

[X86] Simplify the end of custom type legalization for (v2i32/v4i16/v8i8 (bitcast (f64))) by just emitting an EXTRACT_SUBVECTOR instead of a BUILD_VECTOR.

Generic legalization should be able to finish legalizing the EXTRACT_SUBVECTOR probably by turning it into a BUILD_VECTOR. But we should emit the simplest sequence.

llvm-svn: 344424