[MIPS GlobalISel] Register bank select for G_STORE. Select i64 store
Select gprb or fprb when stored value is defined by either:
copy from physical register or
instruction with only one mapping available for that def operand.
Store of integer s64 is handled with narrowScalar when mapping is applied,
produced artifacts are combined away. Manually set gprb to all register
operands of instructions created during narrowScalar.
Differential Revision: https://reviews.llvm.org/D64268