[MIPS GlobalISel] ClampScalar and select pointer G_ICMP

Add narrowScalar to half of original size for G_ICMP.
ClampScalar G_ICMP's operands 2 and 3 to to s32.
Select G_ICMP for pointers for MIPS32. Pointer compare is same
as for integers, it is enough to declare them as legal type.

Differential Revision: https://reviews.llvm.org/D64856

llvm-svn: 366317