[X86] Use X86 instead of X32 as a check prefix in atomic-idempotent.ll. NFC

[X86] Use X86 instead of X32 as a check prefix in atomic-idempotent.ll. NFC

X32 can refer to a 64-bit ABI that uses 32-bit ints, longs, and pointers.

I plan to add gnux32 command lines to this test so this prepares for that.

Also remove some check lines that have a prefix that is not in any run lines.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360642 91177308-0d34-0410-b5e6-96231b3b80d8