[MIPS GlobalISel] Register bank select for G_LOAD. Select i64 load

Select gprb or fprb when loaded value is used by either:
 copy to physical register or
 instruction with only one mapping available for that use operand.

Load 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/D64269

llvm-svn: 365323