Reenable the MergeFunctions pass

The crash that happened in #23566 doesn't happen anymore with the LLVM mergefunc
pass enabled and it hugely reduces code size (for example it shaves off 10% of the
final Servo executable). This patch reenables it.

src/rustllvm/PassWrapper.cpp

@@ -430,8 +430,9 @@ extern "C" void LLVMRustConfigurePassManagerBuilder(
     LLVMPassManagerBuilderRef PMBR, LLVMRustCodeGenOptLevel OptLevel,
     bool MergeFunctions, bool SLPVectorize, bool LoopVectorize, bool PrepareForThinLTO,
     const char* PGOGenPath, const char* PGOUsePath) {
-  // Ignore mergefunc for now as enabling it causes crashes.
-  // unwrap(PMBR)->MergeFunctions = MergeFunctions;
+#if LLVM_RUSTLLVM
+  unwrap(PMBR)->MergeFunctions = MergeFunctions;
+#endif
   unwrap(PMBR)->SLPVectorize = SLPVectorize;
   unwrap(PMBR)->OptLevel = fromRust(OptLevel);
   unwrap(PMBR)->LoopVectorize = LoopVectorize;