[clang] [llvm] [NFC][PowerPC] Add test case for lockdown of vector compare greater than support for Zero vector comparisons (PR #147246)

[via llvm-commits]

Himadhith wrote:

- The assertions for this file `clang/test/CodeGen/PowerPC/check-zero-vector.c` are auto-generated. 
- But for the file `llvm/test/CodeGen/PowerPC/check-zero-vectors.ll`, the generated assertions were very long, I have limited them to the important code blocks. Also changed the register numbers to regular expression, as I had issues of register numbers being changed over different runs.

https://github.com/llvm/llvm-project/pull/147246