Enhancement: optimize `pure` function calls better

[JohanEngelen]

See https://reviews.llvm.org/rL301680

Would this attribute be helpful for pure functions? Or does it imply too much?

[JohanEngelen]

speculatable would be too much on a pure function: pure in D does not mean calls can be introduced that would otherwise not happen. E.g. calling foo(0) for code fragmentif (x != 0) foo(x).

What we can do is apply readnone or readonly to pure functions that don't take pointer arguments (or only immutable pointers?). https://d.godbolt.org/z/i2Dpxw

[kinke]

We also have lovely stuff like void* pureMalloc()(size_t size) @trusted pure @nogc nothrow (core.memory)...

[dnadlinger]

pure != PUREstrong anyway.