MIR: Allow parsing of empty machine functions

If you run llc -stop-after=codegenprepare and feed the resulting MIR
to llc -start-after=codegenprepare, you'll have an empty machine
function since we haven't run any isel yet. Of course, this only works
if the MIRParser believes you that this is okay.

This is essentially a revert of r241862 with a fix for the problem it
was papering over.

llvm-svn: 299975
diff --git a/llvm/test/CodeGen/MIR/Generic/llvmIR.mir b/llvm/test/CodeGen/MIR/Generic/llvmIR.mir
index 432b18f..5c0e60e 100644
--- a/llvm/test/CodeGen/MIR/Generic/llvmIR.mir
+++ b/llvm/test/CodeGen/MIR/Generic/llvmIR.mir
@@ -28,10 +28,8 @@
   IfUnequal:
     ret i32 0
   }
-  
+
 ...
 ---
 name: foo
-body: |
-  bb.0:
 ...