Eliminate IRRoundingModeDFP by merging its values into IRRoundingMode.
Retain encodings. The rationale is that a rounding mode is an abstraction
and as such independent of formats used to represent numeric values.
This was triggered by the need for a rounding mode to express conversions
between binary floating point values and decimal floating point values.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2725 8f6e269a-dfd6-0310-a8e1-e2731360e62c