[PowerPC] Fix assert from machine verify pass that missing undef register flag

Fix assert about using an undefined physical register in machine instruction verify pass. 
The reason is that register flag undef is missing when doing transformation from If Conversion Pass.

```
Bad machine code: Using an undefined physical register 
- function:    func_65
- basic block: %bb.0 entry (0x10024740738)
- instruction: BCLR killed $cr5lt, implicit $lr8, implicit $rm, implicit undef $x3
- operand 0:   killed $cr5lt
LLVM ERROR: Found 1 machine code errors.
```

There are also other existing testcases with same issue. So I add -verify-machineinstrs option to open verifying.

Differential Revision: https://reviews.llvm.org/D55408

llvm-svn: 348566