Make HReg a struct. In the past there were several occurences where

Make HReg a struct. In the past there were several occurences where
a HReg was assigned to an integer. This worked by accident because the
bits representing the register number (which was meant to be accessed)
happened to be in the right place.
Two new functions: hregIsInvalid and sameHReg.
The HReg struct just wraps the integer that was previously used to
represent a register without changing the encoding.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2682