V4 RegAlloc: fix callee saved register range handling

This makes sure that the index of the fixed(FP)RegisterRanges matches
the indexes for normal-/fpRegisters, because this index is used to check
if a chosen register intersects with a fixed (= caller saved) register
at call sites.

Change-Id: Ie31554dbe8ed99cb38ca6b2506da663be41d82f6
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>