[llvm] [PowerPC] Do not generate `isel` instruction if target doesn't have this instruction (PR #72845)

[Kai Luo via llvm-commits]

================
@@ -13,7 +13,7 @@
 ; CHECK: addic 29, 0, 20
 ; Save CR through R12 using R29 as the stack pointer (aligned base pointer).
 ; CHECK: mfcr 12
-; CHECK: stw 12, -24(29)
+; CHECK: stw 12, -28(29)
----------------
bzEq wrote:

It uses one more CSR(`r28`) than the original case.

https://github.com/llvm/llvm-project/pull/72845