Variable auto-init: split out small arrays

Summary: Following up with r355181, initialize small arrays as well.

LLVM stage2 shows a tiny size gain.

<rdar://48523005>

Reviewers: glider, pcc, kcc, rjmccall

Subscribers: jkorous, dexonsmith, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58885

llvm-svn: 355660