[InferAddressSpaces][NFC] Small code improvements for the InferAddressSpaces pass

There is a bunch of code improvements in the patch: marking as const everything what can be
const and fixing some typos in comments.

Also the patch removes the shadowing parameter TTI from the rewriteWithNewAddressSpaces
method, the TTI parameter is not required because the same field is in the class.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D121671