[DebugInfo][X86] Add start-after flags to MIR tests

These tests would fail with -verify-machineinstrs because the MI
generated from the IR would be merged with the one already in the MIR
files, and we get the following error:

```
*** Bad machine code: Function has NoVRegs property but there are VReg operands ***
- function:    f
```

Differential Revision: https://reviews.llvm.org/D49191

llvm-svn: 336907