[DebugInfo][InstrRef] Recover stack-slot tracking performance

This patch is like D111627 -- instead of calculating IDF for every location
on the stack, only do it for the smallest units of interference, and copy
the PHIs for those units to any aliases.

The test added runs placeMLocPHIs directly, and tests that:
 * A def of the lower 8 bits of a stack slot causes all aliasing regs to
   have PHIs placed,
 * It doesn't cause the equivalent location to x86's $ah, which isn't
   aliased, to have a PHI placed.

Differential Revision: https://reviews.llvm.org/D112324