[CGCall] Annotate `this` argument with alignment

Authored by lebedev.ri on Apr 7 2021, 12:29 AM.


[CGCall] Annotate this argument with alignment

As it is being noted in D99249, lack of alignment information on this
has been preventing LICM from happening.

For some time now, lack of alignment attribute does *not* imply
natural alignment, but an alignment of 1.
Also, we used to treat dereferenceable as implying alignment,
but we no longer do, so it's a bugfix.

Differential Revision: https://reviews.llvm.org/D99790