uncprop: small compile time optimization with switches

In the process of converting gswitch away from CASE_LABEL_EXPR,
I found a place in uncprop (like the case in dom) where we store
the whole CASE_LABEL_EXPR. This place only needed to store the value
of the case rather than the whole case expression. This does that
small optimization and adds a few comments for the next person
to understand what is going on here. It was not obvious at my
first read of the code what it was doing or what error_mark
was being used for.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-uncprop.cc (associate_equivalences_with_edges): For switches
	info only store the case low value to be recorded as
	the only value. Add comments.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>