[Tests] Rename some test files for consistency

Most are named "atomic-something" so rename the few which were "atomic_something".  I keep typing the wrong name due to the inconsistency.  :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353956 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/test/CodeGen/X86/atomic_add.ll b/test/CodeGen/X86/atomic-add.ll
similarity index 100%
rename from test/CodeGen/X86/atomic_add.ll
rename to test/CodeGen/X86/atomic-add.ll

diff --git a/test/CodeGen/X86/atomic_idempotent.ll b/test/CodeGen/X86/atomic-idempotent.ll
similarity index 100%
rename from test/CodeGen/X86/atomic_idempotent.ll
rename to test/CodeGen/X86/atomic-idempotent.ll

diff --git a/test/CodeGen/X86/atomic_mi.ll b/test/CodeGen/X86/atomic-mi.ll
similarity index 100%
rename from test/CodeGen/X86/atomic_mi.ll
rename to test/CodeGen/X86/atomic-mi.ll

diff --git a/test/CodeGen/X86/atomic_op.ll b/test/CodeGen/X86/atomic-op.ll
similarity index 100%
rename from test/CodeGen/X86/atomic_op.ll
rename to test/CodeGen/X86/atomic-op.ll