Fix a bug in the register allocator around pair allocation.

We may get hints that do not work with the current implementation
of register pairs, which forces the allocation of (low + 1)
for the high register. For example, if the hint is EBX, we will
allocate ESP for the high register.

bug:23043730

(cherry picked from commit f2975811)

Change-Id: Ib395e36616017a87d3055218d72417f4e9ff6501