[MIR] Change test case to read from stdin instead of file

[MIR] Change test case to read from stdin instead of file

The

    ;CHECK: bb
    ;CHECK-NEXT: %namedVReg1353:_(p0) = COPY $d0

parts of the test case failed when the tests were placed in a directory
including "bb" in the path, since the full path of the file is then
output in the
 ; ModuleID = '/repo/bb/
line which the CHECK matched on and then the CHECK-NEXT failed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371171 91177308-0d34-0410-b5e6-96231b3b80d8