Correctly tag TR_HPR register kind

[fjeremic]

As part of #2737 we've introduced changes to the Z register dependency assignment in which we need to count the number of HPRs which we need to assign as part of the dependency. This code currently relies on having valid register kinds on the virtual registers.

However it seems for HPRs we don't actually properly set the register kind when allocating a virtual register, hence we had to do a workaround to get over this issue. The task here is to remove the ugly hack in commit #2737.