DMD's backend doesn't produce optimal init code for smaller types

[dlangBugzillaToGithub]

Mike Franklin reported this on 2017-11-25T01:46:23Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=18009

Description

An enhancement to DMD's frontend was submitted at https://github.com/dlang/dmd/pull/2146 for smaller types' init to elicit better code gen in the backend.

The argument for the code change in the frontend was one of expediency rather than due diligence.

Some participants in the discussion suggested that such an optimization should be done in the backend.  GDC already optimizes such code in its backend demonstrating feasibility and establishing precedent.  

See https://github.com/dlang/dmd/pull/2146 for context