update_mir_test_checks: Improve the check for LLVM IR in MIR files

The LLVM IR section of a MIR document can start with "--- |" rather
than just "---", because "|" is a sigil for a freeform document in
YAML. We need to handle this so that we don't try to add check lines
to the LLVM IR functions in a MIR file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323178 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed