Quick compiler: generalize NarrowRegLoc()

Some of the RegStorage utilites (DoubleToLowSingle(),
DoubleToHighSingle(), etc.) worked only for targets which
which treat double precision registers as a pair of aliased
single precision registers.

This CL elminates those utilities, and replaces them with
a new RegisterInfo utility that will search an aliased register
set and return the member matching the required storage
configuration (if it exists).

Change-Id: Iff5de10f467d20a56e1a89df9fbf30d1cf63c240