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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255668 91177308-0d34-0410-b5e6-96231b3b80d8