LLVM: Add test for calling reduce with float mode set to optimized

test/cases/float_mode_optimized_reduce.zig

@@ -0,0 +1,12 @@
+pub fn main() void {
+    var a: @Vector(2, f32) = @splat(5.0);
+    _ = &a;
+
+    @setFloatMode(.optimized);
+    var b = @reduce(.Add, a);
+    _ = &b;
+}
+
+// run
+// backend=llvm
+//