[Assignment Tracking][15/*] Account for assignment tracking in simplifycfg

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

Update simplifycfg:
sinkLastInstruction - preserve debug use-before-defs.

SpeculativelyExecuteBB - replace the value component of dbg.assign intrinsics
when stores are hoisted and merged using a select, and don't delete them.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133310