[CodeGen] Only use actual alloca alignment (#178361)
Remove getPrefTypeAlign calls and use only the alloca's explicit
alignment, since the type may not be semantically useful, there is no
useful reason to change alignment to support it.
The alloca's explicit alignment (from getAlign()) is already optimally
correct; we don't need to derive alignment from the allocated type.
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
