When expanding an operand, it's not the result value

When expanding an operand, it's not the result value
type that matters but the operand type.  This fixes
2008-01-08-IllegalCMP.ll which crashed with the new
legalize infrastructure because SETCC with result
type i8 and operand type i64 was being custom expanded
by the X86 backend.  With this fix, the gcc build gets
as far as the first libcall.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46525 91177308-0d34-0410-b5e6-96231b3b80d8