alignment: subtly change meaning of datatype_align (JuliaLang#34473)

Rather than meaning the actual alignment of the object, this now means
the preferred alignment of the object. The actual alignment of any
object is the minimum of this preferred alignment and the alignment
supported by the runtime allocator. This aligns us with how LLVM treats
alignment, and is probably reasonably sensible anyways.

Also tries to audit our existing uses of CreateLoad/CreateStore for
correctness, and upgrade some to include pointer-types.

fixes JuliaLang#32414

(cherry picked from commit 268ac24)

Author: vtjnash