Add certain jemalloc functions to MemoryBuiltins

[XMPPwocky]

This informs LLVM that they return noalias pointers, and allows "dead" allocations to be optimized out.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @huonw (or someone else) soon.

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[huonw]

Hm, it's unclear to me how many special-cased optimisations we want for Rust. Thoughts, @alexcrichton?

[alexcrichton]

I don't think we want to go crazy over time or rely on these optimizations being available, but we already have another optimization (the NullCheckElimination pass) and it seems fine to include this. Patches that don't hinder reliance on upstream or stock LLVM seem fine to me in general.