Build correct coercion types in SparcV9ABIInfo.

The coercion type serves two purposes:

 1. Pad structs to a multiple of 64 bits, so they are passed
    'left-aligned' in registers.

 2. Expose aligned floating point elements as first-level elements, so
    the code generator knows to pass them in floating point registers.

We also compute the InReg flag which indicates that the struct contains
aligned 32-bit floats. This flag is used by the code generator to pick
the right registers.