splitMergedValStore will split a store into two if target prefers this, or if
-force-split-store is passed.
This patch adds the missing handling for endianness in this function along
with a test case.
Review: Eli Friedman
https://reviews.llvm.org/D44396
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327375 91177308-0d34-0410-b5e6-96231b3b80d8
