[MIPS GlobalISel] Select bitreverse. Recommit

[MIPS GlobalISel] Select bitreverse. Recommit

G_BITREVERSE is generated from llvm.bitreverse.<type> intrinsics,
clang genrates these intrinsics from __builtin_bitreverse32 and
__builtin_bitreverse64.
Add lower and narrowscalar for G_BITREVERSE.
Lower G_BITREVERSE on MIPS32.

Recommit notes:
Introduce temporary variables in order to make sure
instructions get inserted into MachineFunction in same order
regardless of compiler used to build llvm.

Differential Revision: https://reviews.llvm.org/D71363