[GlobalISel] CombinerHelper: Fix a bug in matchCombineCopy

Summary:
When combining COPY instructions, we were replacing the destination registers
with the source register without checking register constraints. This patch adds
a simple logic to check if the constraints match before replacing registers.

Reviewers: qcolombet, aditya_nandakumar, aemerson, paquette, dsanders, Petar.Avramovic

Reviewed By: aditya_nandakumar

Subscribers: rovka, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70616