Add global setter for value/mask pair for SSA names.

Add global setter for value/mask pair for SSA names.

This patch provides a way to set the value/mask pair of known bits
globally, similarly to how we can use set_nonzero_bits for known 0
bits.  This can then be used by CCP and IPA to set value/mask info
instead of throwing away the known 1 bits.

In further clean-ups, I will see if it makes sense to remove
set_nonzero_bits altogether, since it is subsumed by value/mask.

gcc/ChangeLog:

	* tree-ssanames.cc (set_bitmask): New.
	* tree-ssanames.h (set_bitmask): New.