[VTA, PR41473] drop NULL locations from lists

[Alexandre Oliva]

Entries in location lists whose RTL is NULL or not representable end up
causing us to waste debug info space emitting empty DW_AT_location.

This patch optimizes the debug information, omitting the useless
DW_AT_location.  This should work around the bug in Darwin's dsymutil.

While at that, I arranged for entries in location lists that are not
representable in debug info to not waste an entry in the output location
list.

The patch also fixes the handling of CONST_DOUBLEs and CONST_VECTORs
within CONCAT and CONCATN, that was currently disabled because mode was
specified as VOIDmode by the handlers of these RTL forms.  Even with
-gdwarf-4, we'd still have failed to emit them.

Ok to install if this passes regstrap?

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-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer