[llvm] [PowerPC ][NFC] Add a test case for the function atomic_compare_exchange_weak (PR #141263)

[Amy Kwan via llvm-commits]

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@@ -0,0 +1,100 @@
+; Test the generation of asm for the function:
+; int foo(_Atomic int *cp, int *old, int c) {
+;  return atomic_compare_exchange_weak_explicit(cp, old, c, __ATOMIC_RELAXED, __ATOMIC_RELAXED);
+; }  
+
+; RUN: llc < %s -ppc-asm-full-reg-names  -mtriple=powerpc-ibm-aix -mcpu=pwr8  -verify-machineinstrs | FileCheck %s --check-prefix=CHECK
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amy-kwan wrote:

nit: split the run lines since they seem too long

https://github.com/llvm/llvm-project/pull/141263