llc: Add ability to parse mir from stdin

- Add -x <language> option to switch between IR and MIR inputs.
- Change MIR parser to read from stdin when filename is '-'.
- Add a simple mir roundtrip test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304825 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed