Make the x86 backend custom-lower UINT_TO_FP and FP_TO_UINT on 32-bit

systems instead of attempting to promote them to a 64-bit SINT_TO_FP or 
FP_TO_SINT.  This is in preparation for removing the type legalization 
code from LegalizeDAG: once type legalization is gone from LegalizeDAG, 
it won't be able to handle the i64 operand/result correctly.

This isn't quite ideal, but I don't think any other operation for any 
target ends up in this situation, so treating this case specially seems 
reasonable.

llvm-svn: 72324