Undo an overly aggression application of unsigned to register numbers in 8aa39661 (https://codereview.chromium.org/1676123002).

Now, instead of -verbose=regalloc output like:

++++++ Unhandled:
R=jpp@chromium.org, kschimpf@google.com, 4294967295  V=%__4  Range=[2:7), 4294967295  V=%__5  Range=[7:8), 4294967295  V=%__6  Range=[9:11), -1  V=%__4  Range=[2:7), -1  V=%__5  Range=[7:8), -1  V=%__6  Range=[9:11)

we have the originally intended:

++++++ Unhandled:

BUG= none

Review URL: https://codereview.chromium.org/1868543002 .