[amdgpu][nfc] Allocate kernel-specific LDS struct deterministically

A kernel may have an associated struct for laying out LDS variables.
This patch puts that instance, if present, at a deterministic address by
allocating it at the same time as the module scope instance.

This is relatively likely to be where the instance was allocated anyway (~NFC)
but will allow later patches to calculate where a given field can be found,
which means a function which is only reachable from a single kernel will be
able to access a LDS variable with zero overhead. That will be particularly
helpful for applications that instantiate a function template containing LDS
variables once per kernel.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D127052