[X86] Replace action Promote with Expand for operation ISD::SINT_TO_FP

Summary:
If attribute "use-soft-float"="true" is set then X86ISelLowering.cpp sets
'Promote' action for ISD::SINT_TO_FP operation on type i32.

But 'Promote' action is not proper in this case since lib function
__floatsidf is available for casting from signed int to float type.
Thus Expand action is more suitable here.

The Expand action should be set for ISD::UINT_TO_FP for soft float as well.

If function attribute "use-soft-float"="true" is set then infinite looping
can happen in DAG combining, function visitSINT_TO_FP() replaces SINT_TO_FP
node with UINT_TO_FP node and function combineUIntToFP() replace vice versa in cycle.
The fix prevents it.

Patch by vrybalov

Differential Revision: https://reviews.llvm.org/D45572

llvm-svn: 330711