Resolves#169701.
This PR extends the existing InstCombine operation which folds `tbl1`
intrinsics to `shufflevector` if the mask operand is constant. Before
this change, it only handled 64-bit `tbl1` intrinsics with no
out-of-bounds indices. I've extended it to support both 64-bit and
128-bit vectors, and it now handles the full range of `tbl1`-`tbl4` and
`tbx1`-`tbx4`, as long as at most two of the input operands are actually
indexed into.
For the purposes of `tbl`, we need a dummy vector of zeroes if there are
any out-of-bounds indices, and for the purposes of `tbx`, we use the
"fallback" operand. Both of those take up an operand for the purposes of
`shufflevector`.
This works a lot like https://github.com/llvm/llvm-project/pull/169110,
with some added complexity because we need to handle multiple operands.
I raised a couple questions in that PR that still need to be answered:
- Is it correct to check `IsA<UndefValue>` for each mask index, and set
the output mask index to -1 if so? This is later folded to a poison
value, and I'm not sure about the subtle differences between poison and
undef and when you can substitute one for the other. As I mentioned in
#169110, the existing x86 pass (`simplifyX86vpermilvar`) already behaves
this way when it comes to undef.
- How can I write an Alive2 proof for this? It's very hard to find good
documentation or tutorials about Alive2.
As with #169110, most of the regression test cases were generated using
Claude. Everything else was written by me.
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