Derive alignment from `llvm.assume` outside the use/def chain.

[llvmbot]

Bugzilla Link	22049
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@hfinkel

Extended Description

The issue is explained in full with an example in [1]. In short, the issue arises when articulating the alignment of a variable/zero sized array in a struct. In this case, loads and stores from the array don't share a use/def chain with alignment fact.

[1] http://llvm.1065342.n5.nabble.com/Correct-usage-of-llvm-assume-for-loop-vectorization-alignment-td75757.html

[nikic]

The link is dead, so this issue no longer actionable. There have been various improvements to alignment inference since that time...