[Sparc] Fix handling of double incoming arguments on sparc little-endian.

On SparcV8, doubles get passed in two 32-bit integer registers. The call
code was already handling endianness correctly, but the incoming
argument code was not -- it got the two halves in opposite order.

Also remove some dead code in LowerFormalArguments_32 to handle
less-than-32bit values, which can't actually happen.

Finally, add some test cases for the 32-bit calling convention, cribbed
from the 64abi.ll test, and run for both big and little-endian.

llvm-svn: 255668