Tidy RegStorage for X86.

Don't use global variables initialized in constructors to hold onto constant
values, instead use the TargetReg32 helper. Improve this helper with the use
of lookup tables. Elsewhere prefer to use constexpr values as they will have
less runtime cost.
Add an ostream operator to RegStorage for CHECK_EQ and use.

Change-Id: Ib8d092d46c10dac5909ecdff3cc1e18b7e9b1633