Keep decorations when replacing loads in access-chain-convert. (#1829)

In local-access-chain-convert, we replace loads by load the entire
variable, then doing the extract.  The extract will have the same value
as the load.  However, if the load has a decoration on it, the
decoration is lost because we do not copy any them to the new id.

This is fixed by rewritting the load into the extract and keeping the
same result id.

This change has the effect that we do not call DCEInst on the loads
because the load is not being deleted, but replaced.  This could leave
OpAccessChain instructions around that are not used.  This is not a
problem for -O and -Os.  They run local_single_*_elim passes and then
dead code elimination.  The dce will remove the unused access chains,
and the load elimination passes work even if there are unused access
chains.  I have added test to them to ensure they will not loss
opportunities.

Fixes #1787.