When we compare a range against a constant for equality or inequality, 
there is currently no attempt made to utilize the known bits.

This patch adds a method to the irange_bitmask class to ask if a 
specific value satisfies the known bit pattern.  Operators equal and 
not_equal then utilize it when comparing to a constant eliiminating a 
class of cases we don;t currently get. ie.

if (x & 1) return;
if (x == 97657) foo()

will eliminate the call to foo, even though we do not remove all the odd 
numbers from the range.  THe bit pattern comparison for
   [irange] unsigned int [0, 0] [2, +INF] MASK 0xfffffffe VALUE 0x1  
will indicate that any even constants will be false.

Bootstraps on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew