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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242174
91177308-0d34-0410-b5e6-
96231b3b80d8
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