Quick compiler: allocate doubles as doubles

Significant refactoring of register handling to unify usage across
all targets & 32/64 backends.

Reworked RegStorage encoding to allow expanded use of
x86 xmm registers; removed vector registers as a separate
register type.  Reworked RegisterInfo to describe aliased
physical registers.  Eliminated quite a bit of target-specific code
and generalized common code.

Use of RegStorage instead of int for registers now propagated down
to the NewLIRx() level.  In future CLs, the NewLIRx() routines will
be replaced with versions that are explicit about what kind of
operand they expect (RegStorage, displacement, etc.).  The goal
is to eventually use RegStorage all the way to the assembly phase.

TBD: MIPS needs verification.
TBD: Re-enable liveness tracking.

Change-Id: I388c006d5fa9b3ea72db4e37a19ce257f2a15964