[MIPS GlobalISel] Select population count (popcount)

G_CTPOP is generated from llvm.ctpop.<type> intrinsics, clang generates
these intrinsics from __builtin_popcount and __builtin_popcountll.
Add lower and narrow scalar for G_CTPOP.
Lower G_CTPOP for MIPS32.

Differential Revision: https://reviews.llvm.org/D73216