[M68k] Add instruction selection support for zext with PCD addressing

Instruction selection was failing when trying to zero extend a value
loaded from a PC-relative address. This adds support for zero extension
using the "program counter indirect with displacement" addressing mode.
It also adds a test with code that was previously failing to compile.

This fixes a compile error in Rust's libcore.

Differential Revision: https://reviews.llvm.org/D149034