[LegalizeTypes] Teach SoftenFloatOp_SELECT_CC to handle operand 2 or 3 being softened.

This can only happen on X86 when fp128 is a legal type, but we
go through softening to generate libcalls. This causes fp128 to
be softened to fp128 instead of an integer type. This can be
removed if D67128 lands.

llvm-svn: 371493