[GlobalOpt] Generalize malloc-to-global for any allocation function

We can generalize the malloc-to-global transform for other allocation functions which are both a) removable, and b) have a known initialization value.

One subtlety that I want to point out - mostly because I hadn't realized it was true until I took a closer look - is that the existing code doesn't prove that initialization/malloc happens only once. The initialization function can be called multiple times. This is correct without special handling for malloc as undef can map to any value previously written, but a non-undef initializing allocation it means we may end up memseting the new global repeatedly. In particular, this means it's not legal to fold the memset into the initializer of the global.

Differential Revision: https://reviews.llvm.org/D117503