Handle *_EXTEND_VECTOR_INREG during Integer Legalization

Summary:
These nodes need legalization for 3-element vectors.  This commit
handles the legalization and adds tests for zext and sext.

This fixes PR30614.

Reviewers: RKSimon, srhines

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25268

llvm-svn: 283496