[llvm] [UpdateTestChecks] Don't fail silently when conflicting CHECK lines means no checks are generated for some functions (PR #159321)

[Alexander Richardson via llvm-commits]

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@@ -0,0 +1,7 @@
+# REQUIRES: riscv-registered-target
+
+# RUN: cp -f %S/Inputs/conflicting-prefixes.ll %t.ll
+# RUN: %update_llc_test_checks --no-generate-body-for-unused-prefixes %t.ll 2>&1 | FileCheck %s
+# RUN: diff -u %S/Inputs/conflicting-prefixes.ll.expected %t.ll
+
+# CHECK: WARNING: For function 'foo', the following RUN lines will not generate checks due to conflicting output
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arichardson wrote:

Should we check the full warning message here?

https://github.com/llvm/llvm-project/pull/159321