[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.

llvm-svn: 371171