[AMDGPU] Add more verbose logs to PromoteAlloca

More specifically make it more talkative when it's looking at the users of
an alloca to promote it to a vector.

A common failure point of the pass is unknown or weird users of the alloca.
While debugging issues related to this pass one of the first thing I usually
did was to add logs to see how the users were being handled.
Having such logs in directly seems to be a nice addition.

Reviewed By: arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D148629