[MIPS} Address ISel failures for 64 bit fpus in microMIPS

Add the instructions and patterns for loads and stores in microMIPSr3
when a 64 bit FPU is present. Previously, this would lead to an
instruction selection failure.

This resolves PR/49200.

Thanks to jdeguire for reporting the issue!

Differential Revision: https://reviews.llvm.org/D124723