[MTE] Pin the tagged base pointer to one of the stack slots.

Summary:
Pin the tagged base pointer to one of the stack slots, and (if
necessary) rewrite tag offsets so that an object that occupies that
slot has both address and tag offsets of 0. This allows ADDG
instructions for that object to be eliminated and their uses replaced
with the tagged base pointer itself.

This optimization must be done in machine instructions and not in the IR
instrumentation pass, because referring to a stack slot through an IRG
pointer would confuse the stack coloring pass.

The optimization makes a (pretty naive) attempt to find the slot that
would benefit the most by counting the uses of stack slots in the
function.

Reviewers: ostannard, pcc

Subscribers: merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72365