Implement first D-specific optimization pass: -simplify-drtcalls. It uses the machinery of the standard -simplify-libcalls pass, but optimizes calls to the D runtime instead of calls to C libraries. At the moment, these optimizations are implemented by this pass: - Avoid the runtime call for `arr.length = newlen` if it can determine that the new length isn't longer than the old one. - Ditto for `cast(T[]) arr` if it will clearly always succeed. (e.g. if the length of the original array is zero, or if the old element size is a multiple of the new element size)

module bug1;
struct Foo { Foo opSub(ref Foo b) { return Foo(); } }
struct Bar { Foo whee; }
void test(ref Bar moo) { Foo nngh; auto plonk = nngh - moo.whee; }
void main() { Bar bar; test(bar); }