Build an ordered vector of register mask operands (i.e., calls) when
computing live intervals. Provide a checkRegMaskInterference() function
that computes a bit mask of usable registers for a live range.

This is a quick way of determining of a live range crosses any calls,
and restricting it to the callee saved registers if it does.
Previously, we had to discover call clobbers for each candidate register
independently.

llvm-svn: 150077