Rewrite LowerCallTo and Select(ISD::CALL) to properly handle float varargs

Tell the SelectionDAG ISel to expand SEXTLOAD of i1 and i8, rather than
  complicate the code in ISD::SEXTLOAD to do it by hand
Combine the FP and Int ISD::LOAD codegen
Generate better code for constant pool loads

As a result, all of Shootout, and likely many other programs are now
working.

llvm-svn: 20945