(fixup_var_refs{,_insns}): Add new parms for promoted mode and signedness;

all callers changed.
Insert mode conversions when required.
(fixup_var_refs_1): Add new parms for mode and signedness; handle SUBREGs
for promoted values.
(assign_parms): Promote the mode of parameters when useful for the target
machine.

From-SVN: r2175