s390: Fix HReg <--> UInt mixups.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2657 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Fix HReg <--> UInt mixup.
vreg is a HReg, so we should not assign a hregNumber() to it.
Introduce and use vregno instead.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2658 8f6e269a-dfd6-0310-a8e1-e2731360e62c