rust_llvm: Add way to reflectively ask if a ValueRef is a known constant int.

Add option-returning variants to `const_to_int`/`const_to_uint` that
never assert fail. (These will be used for overflow checking from
rustc_trans::trans::consts.)

src/librustc_llvm/lib.rs

@@ -1976,6 +1976,7 @@ pub fn LLVMDICompositeTypeSetTypeArray(Builder: DIBuilderRef,
     pub fn LLVMIsAArgument(value_ref: ValueRef) -> ValueRef;
 
     pub fn LLVMIsAAllocaInst(value_ref: ValueRef) -> ValueRef;
+    pub fn LLVMIsAConstantInt(value_ref: ValueRef) -> ValueRef;
 
     pub fn LLVMInitializeX86TargetInfo();
     pub fn LLVMInitializeX86Target();

src/librustc_trans/trans/common.rs

@@ -963,6 +963,32 @@ pub fn const_to_uint(v: ValueRef) -> u64 {
     }
 }
 
+fn is_const_integral(v: ValueRef) -> bool {
+    unsafe {
+        !llvm::LLVMIsAConstantInt(v).is_null()
+    }
+}
+
+pub fn const_to_opt_int(v: ValueRef) -> Option<i64> {
+    unsafe {
+        if is_const_integral(v) {
+            Some(llvm::LLVMConstIntGetSExtValue(v))
+        } else {
+            None
+        }
+    }
+}
+
+pub fn const_to_opt_uint(v: ValueRef) -> Option<u64> {
+    unsafe {
+        if is_const_integral(v) {
+            Some(llvm::LLVMConstIntGetZExtValue(v))
+        } else {
+            None
+        }
+    }
+}
+
 pub fn is_undef(val: ValueRef) -> bool {
     unsafe {
         llvm::LLVMIsUndef(val) != False