[MIPS GlobalISel] Select bswap

G_BSWAP is generated from llvm.bswap.<type> intrinsics, clang genrates
these intrinsics from __builtin_bswap32 and __builtin_bswap64.
Add lower and narrowscalar for G_BSWAP.
Lower G_BSWAP on MIPS32, select G_BSWAP on MIPS32 revision 2 and later.

Differential Revision: https://reviews.llvm.org/D71362