Heed ABIArgInfo::getInReg() for return values.

The 'inreg' attribute can also be applied to function return values in
LLVM IR. The SPARC v9 backend is using the flag when returning structs
containing 32-bit floats.
Fix CoerceIntOrPtrToIntOrPtr on big-endian targets.

Type coercion for argument passing is equivalent to storing the source
type and loading the destination type from the same pointer. On
big-endian targets, this means that the high bits of integers are
preserved.

This patch fixes the CoerceIntOrPtrToIntOrPtr() function on big-endian
targets by inserting the required shift instructions to preserve the
high bits instead of the low bits.

This is used by SparcABIInfo when passing small structs in the high bits
of registers.