[DAG] Fix constant store folding to handle non-byte sizes.

Avoid crashes from zero-byte values due to sub-byte store sizes.

Reviewers: uabelho, courbet, rnk

Reviewed By: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58626

llvm-svn: 354884