Fix tests that rely on LLVM IR verification

Pass -Z verify-llvm-ir to tests that rely on it, to make sure they
pass regardless of the value of verify-llvm-ir in config.toml.

Also remove the 109681.rs test, because it is a duplicat of
common-linkage-non-zero-init.rs.

Author: nikic

tests/crashes/109681.rs

@@ -1,4 +1,4 @@
-//@ compile-flags: -g -Copt-level=0
+//@ compile-flags: -g -Copt-level=0 -Z verify-llvm-ir
 //@ known-bug: #34127
 //@ only-x86_64
 

tests/crashes/34127.rs

@@ -2,6 +2,7 @@
 //@ failure-status: 101
 //@ known-bug: #109681
 //@ ignore-wasm32 this appears to SIGABRT on wasm, not fail cleanly
+//@ compile-flags: -Z verify-llvm-ir
 
 // This test verifies that we continue to hit the LLVM error for common linkage with non-zero
 // initializers, since it generates invalid LLVM IR.