RegAllocGreedy: Fix detection of lanes read by a bundle

SplitKit creates questionably formed bundles of copies
when it needs to copy a subset of live lanes and can't do
it with a single subregister index. These are merely marked
as part of a bundle, and don't start with a BUNDLE instruction.
Queries for the slot index would give the first copy in the
bundle, and we need to inspect the operands of all the other
bundled copies.

Also fix and simplify detection of read lane subsets. This causes
some RISCV test regressions, but these look like accidentally beneficial
splits. I don't see a subrange based reason to perform these splits.

Avoids some really ugly regressions in a future patch.

https://reviews.llvm.org/D146859