[InstCombine] Enhance deduction of alignment for aligned_alloc

This patch improves https://reviews.llvm.org/D76971 (Deduce attributes for aligned_alloc in InstCombine) and implements "TODO" item mentioned in the review of that patch.

> The function aligned_alloc() is the same as memalign(), except for the added restriction that size should be a multiple of alignment.

Currently, we simply bail out if we see a non-constant size - change that.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D100785