Set only fast-math flags which don't produce poison

Author: saethlin

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -420,12 +420,15 @@ extern "C" LLVMAttributeRef LLVMRustCreateMemoryEffectsAttr(LLVMContextRef C,
   }
 }
 
-// Enable a fast-math flag
+// Enable fast-math flags which do not let operations return poison.
 //
 // https://llvm.org/docs/LangRef.html#fast-math-flags
 extern "C" void LLVMRustSetFastMath(LLVMValueRef V) {
   if (auto I = dyn_cast<Instruction>(unwrap<Value>(V))) {
-    I->setFast(true);
+    I->setHasAllowReassoc(true);
+    I->setHasAllowContract(true);
+    I->setHasAllowReciprocal(true);
+    I->setHasNoSignedZeros(true);
   }
 }