putGST_masked: correctly handle the case where the mask is for
FPSCR.RN or FPSCR.DRN, but does not cover the entire field.  Then it
is important to update the exposed parts but leave the not-exposed
parts unchanged.  This is a regression relative to circa 5 years ago.



git-svn-id: svn://svn.valgrind.org/vex/trunk@2929 8f6e269a-dfd6-0310-a8e1-e2731360e62c