[GlobalISel][Localizer] Allow localization of G_INTTOPTR and chains of instructions.

[GlobalISel][Localizer] Allow localization of G_INTTOPTR and chains of instructions.

G_INTTOPTR can prevent the localizer from moving G_CONSTANTs, but since it's
essentially a side effect free cast instruction we can remat both instructions.
This patch changes the localizer to enable localization of the chains by
iterating over the entry block instructions in reverse order. That way, uses will
localized first, and then the defs are free to be localized as well.

This also changes the previous SmallPtrSet of localized instructions to use a
SetVector instead. We're dealing with pointers and need deterministic iteration
order.

Overall, this change improves ARM64 -O0 CTMark code size by around 0.7% geomean.

Differential Revision: https://reviews.llvm.org/D63630

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364001 91177308-0d34-0410-b5e6-96231b3b80d8