change selectiondag to add the sign extended versions of immediate operands

change selectiondag to add the sign extended versions of immediate operands
to instructions instead of zero extended ones.  This makes the asmprinter
print signed values more consistently.  This apparently only really affects
the X86 backend.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81265 91177308-0d34-0410-b5e6-96231b3b80d8