Use the right floating point load/store instructions in PPCInstrInfo::foldMemoryOperandImpl().

The PowerPC floating point registers can represent both f32 and f64 via the
two register classes F4RC and F8RC. F8RC is considered a subclass of F4RC to
allow cross-class coalescing. This coalescing only affects whether registers
are spilled as f32 or f64.

Spill slots must be accessed with load/store instructions corresponding to the
class of the spilled register. PPCInstrInfo::foldMemoryOperandImpl was looking
at the instruction opcode which is wrong.

X86 has similar floating point register classes, but doesn't try to fold
memory operands, so there is no problem there.

llvm-svn: 97262