3824017f8e
Do not emit alloca for ZST locals with multiple assignments
This extends 35566bfd7d to additionally stop emitting unnecessary allocas for zero sized locals that are assigned multiple times.
When rebuilding the standard library with `-Zbuild-std` this reduces the number of locals that require an allocation from 62315 to 61767.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.