[PATCH] Check for undefined before not returning a constant value

[Andrew MacLeod]

Full comments in the PR, but basically substitute and fold was expecting 
to see a constant returned for any range which globally evaluates to a 
constant.
This allowed it to replace all uses of an ssa name as they were encountered.

The ranger model can return UNDEFINED for the range of names in blocks 
which are unreachable..  the patch simply overrides the UNDEFINED value 
in folders query if the global value is a constant so that 
subst_and_fold will get what it is expecting.

Bootstrapped on  x86_64-pc-linux-gnu, no regressions, pushed.

Andrew
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