properly handle the case when LLVM does not have min/maxnum

Author: gnzlbg

src/librustc_trans/builder.rs

@@ -921,15 +921,19 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         self.count_insn("minnum");
         unsafe {
             let instr = llvm::LLVMRustBuildMinNum(self.llbuilder, lhs, rhs);
-            bug!("LLVMRustBuildMinNum is not available in LLVM version < 6.0");
+            if instr.is_null() {
+                bug!("LLVMRustBuildMinNum is not available in LLVM version < 6.0");
+            }
             instr
         }
     }
     pub fn maxnum(&self, lhs: ValueRef, rhs: ValueRef) -> ValueRef {
         self.count_insn("maxnum");
         unsafe {
             let instr = llvm::LLVMRustBuildMaxNum(self.llbuilder, lhs, rhs);
-            bug!("LLVMRustBuildMaxNum is not available in LLVM version < 6.0");
+            if instr.is_null() {
+                bug!("LLVMRustBuildMaxNum is not available in LLVM version < 6.0");
+            }
             instr
         }
     }