[AArch64] Fix assertion failure on widened f16 BUILD_VECTOR

Summary:
Ensure that NormalizedBuildVector returns a BUILD_VECTOR with operands of the
same type. This fixes an assertion failure in VerifySDNode.

Reviewers: SjoerdMeijer, t.p.northover, javed.absar

Reviewed By: SjoerdMeijer

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D50202

llvm-svn: 339013