[Assignment Tracking] Downgrade dbg.assigns to dbg.values in mem2reg

For fully promoted variables dbg.assigns and dbg.values convey the same
information and can be used interchangeably. This patch converts dbg.assigns to
dbg.values for variables promoted by mem2reg. This reduces resource usage by
reducing the amount of unnecessary function local metadata. The compile time
tracker reports that CTMark projects build with LTO-O3-g with 0.4% fewer
instructions retired and peak memory usage is reduced by 2.2%.

Reviewed By: jryans

Differential Revision: https://reviews.llvm.org/D145511