[llvm] [PowerPC] simplify the asm of scmp builtin (PR #200024)

[zhijian lin via llvm-commits]

diggerlin wrote:

> Why does this first let the SCMP node expand and then do a DAG to DAG lowering of the expansion? Why is not being directly lowered as Custom?

good idea, let me try.

https://github.com/llvm/llvm-project/pull/200024