[LoopIdiom] Move X86 specific atomic memcpy test to the X86 directory

Patch https://reviews.llvm.org/rL304806 was causing failures in Aarch64
and multiple other targets since the test should be run on X86 only.

Specifying the target triple is not enough. Moving the testcase to the
X86 target directory in LoopIdiom.

llvm-svn: 304809