[MIPS GlobalISel] Register bank select for G_PHI. Select i64 phi

[MIPS GlobalISel] Register bank select for G_PHI. Select i64 phi

Select gprb or fprb when def/use register operand of G_PHI is
used/defined by either:
 copy to/from physical register or
 instruction with only one mapping available for that use/def operand.

Integer s64 phi 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/D64351

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365494 91177308-0d34-0410-b5e6-96231b3b80d8