Fix handling of double precision loads and stores when Mips1 is targeted.

Mips1 does not support double precision loads or stores, therefore two single
precision loads or stores must be used in place of these instructions. This 
patch treats double precision loads and stores as if they are legal
instructions until MCInstLowering, instead of generating the single precision
instructions during instruction selection or Prolog/Epilog code insertion.

Without the changes made in this patch, llc produces code that has the same 
problem described in r137484 or bails out when
MipsInstrInfo::storeRegToStackSlot or loadRegFromStackSlot is called before
register allocation.

llvm-svn: 137711