implement and document support for filecheck variables. This

implement and document support for filecheck variables.  This
allows matching and remembering a string and then matching and
verifying that the string occurs later in the file.

Change X86/xor.ll to use this in some cases where the test was
checking for an arbitrary register allocation decision.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82891 91177308-0d34-0410-b5e6-96231b3b80d8