X86: Refactor type-splitting to target-legal size vector to a helper function

Summary: This is a preparatory step for D41811: refactoring code for breaking vector operands of binary operation to legal-types.

Reviewers: RKSimon, craig.topper, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41925

llvm-svn: 322296