Add intrinsics for float arithmetic with fast flag enabled

`fast` a.k.a UnsafeAlgebra is the flag for enabling all "unsafe"
(according to llvm) float optimizations.

See LangRef for more information http://llvm.org/docs/LangRef.html#fast-math-flags

Providing these operations with less precise associativity rules (for
example) is useful to numerical applications.

For example, the summation loop:

    let sum = 0.;
    for element in data {
        sum += *element;
    }

Using the default floating point semantics, this loop expresses the
floats must be added in a sequence, one after another. This constraint
is usually completely unintended, and it means that no autovectorization
is possible.

src/rustllvm/RustWrapper.cpp

@@ -164,6 +164,11 @@ extern "C" void LLVMRemoveFunctionAttrString(LLVMValueRef fn, unsigned index, co
                                           to_remove));
 }
 
+// enable fpmath flag UnsafeAlgebra
+extern "C" void LLVMRustSetHasUnsafeAlgebra(LLVMValueRef Instr) {
+    unwrap<Instruction>(Instr)->setHasUnsafeAlgebra(true);
+}
+
 extern "C" LLVMValueRef LLVMBuildAtomicLoad(LLVMBuilderRef B,
                                             LLVMValueRef source,
                                             const char* Name,