[FastIsel][X86] Add support for lowering the first 8 floating-point arguments.

Recommit with fixed argument attribute checking code, which is required to bail
out of all the cases we don't handle yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210815 91177308-0d34-0410-b5e6-96231b3b80d8