[ARM] Fix ARMCodeGenPrepare test cases

While working on FileCheck producing better diagnostics in D53710, I noticed
that our test case is broken in a few different ways. The test was running, but
results were not checked as prefix CHECK-COMMON wasn't defined (which is what
FileCheck should warn about). Also, the output was different in 2 cases because
of recent changes in ARMCodeGenPrepare.

Differential Revision: https://reviews.llvm.org/D53746

llvm-svn: 345386