[AMDGPU] Add implicit vcc_lo on S_CBRANCH_VCCNZ in wave32

* Update skip-if-dead.ll with tests for wave32.
* Fix the crash in verifier in one newly enabled test by adding
  missing fixImplicitOperands in branch insertion code.

```
*** Bad machine code: Using an undefined physical register ***
- function:    test_kill_divergent_loop
- basic block: %bb.2 bb (0xad96308)
- instruction: S_CBRANCH_VCCNZ %bb.1, implicit $vcc_lo
- operand 1:   implicit $vcc_lo
LLVM ERROR: Found 1 machine code errors.
```

* Simplify "cbranch_kill" to not use interp instructions.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D96793

lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -2415,6 +2415,7 @@ unsigned SIInstrInfo::insertBranch(MachineBasicBlock &MBB,
   MachineInstr *CondBr =
     BuildMI(&MBB, DL, get(Opcode))
     .addMBB(TBB);
+  fixImplicitOperands(*CondBr);
   BuildMI(&MBB, DL, get(AMDGPU::S_BRANCH))
     .addMBB(FBB);