Read global value/mask in IPA.

Read global value/mask in IPA.

Instead of reading the known zero bits in IPA, read the value/mask
pair which is available.

There is a slight change of behavior here.  I have removed the check
for SSA_NAME, as the ranger can calculate the range and value/mask for
INTEGER_CST.  This simplifies the code a bit, since there's no special
casing when setting the jfunc bits.  The default range for VR is
undefined, so I think it's safe just to check for undefined_p().

gcc/ChangeLog:

	* ipa-prop.cc (ipa_compute_jump_functions_for_edge): Read global
	value/mask.

gcc/testsuite/ChangeLog:

	* g++.dg/ipa/pure-const-3.C: Move source to...
	* g++.dg/ipa/pure-const-3.h: ...here, and adjust original test
	accordingly.
	* g++.dg/ipa/pure-const-3b.C: New.