[mips] Enhance command line option "-mno-ldc1-sdc1" to expand base+index double

precision loads and stores as well as reg+imm double precision loads and stores.

Previously, expansion of loads and stores was done after register allocation,
but now it takes place during legalization. As a result, users will see double
precision stores and loads being emitted to spill and restore 64-bit FP registers.

llvm-svn: 190235