AMDGPU/SI: Fix read2 merging into a super register.

AMDGPU/SI: Fix read2 merging into a super register.

If the read2 produced was supposed to be writing into a
super register, it would use the wrong subregister indices.
Fix this by inserting copies, so we only ever write to a vreg_64.
Run the register coalescer again to clean this up, although this
isn't ideal and often does result in an extra move.

Also remove the assert that offset1 > offset0.

There isn't a real reason to not allow this other than a minor
convenience in the compiler, and it doesn't seem worth the effort
of avoiding it.

llvm-svn: 242174