[llvm] [RISCV][GlobalISel] Legalize G_ADD, G_SUB, G_AND, G_OR, G_XOR on RISC-V Vector Extension (PR #71400)

[Jiahan Xie via llvm-commits]

jiahanxie353 wrote:

I found that to get things working, I should be using 
```C++
{
        nxv1s8,  nxv2s8,   nxv4s8,  nxv8s8,   nxv16s8,  nxv32s8, nxv64s8, nxv1s16,
        nxv2s16, nxv4s16,  nxv8s16, nxv16s16, nxv32s16, nxv1s32, nxv2s32, nxv4s32,
        nxv8s32, nxv16s32, nxv1s64, nxv2s64,  nxv4s64,  nxv8s64
}
```
in-place instead of declaring it as an `std::initializer_list<LLT>` because otherwise the tests are just failing.
Something like 
```cpp
legalFor([=, &ST](const LegalityQuery &Query) {
  return ST.hasVInstruction() && typeInSet(0, {
        nxv1s8,  nxv2s8,   nxv4s8,  nxv8s8,   nxv16s8,  nxv32s8, nxv64s8, nxv1s16,
        nxv2s16, nxv4s16,  nxv8s16, nxv16s16, nxv32s16, nxv1s32, nxv2s32, nxv4s32,
        nxv8s32, nxv16s32, nxv1s64, nxv2s64,  nxv4s64,  nxv8s64
})(Query) &&
             (Query.Types[0].getScalarSize() != 64 || ST.hasVInstruction64()) &&
             (Query.Types[0].getElementCount().getKnownMinValue() != 1 || ST.getELEN() == 64));
}
```
works but not
```c++
.legalFor([=, &ST](const LegalityQuery &Query) {
  return ST.hasVInstruction() && typeInSet(0, AllVecTys)(Query) &&
             (Query.Types[0].getScalarSize() != 64 || ST.hasVInstruction64()) &&
             (Query.Types[0].getElementCount().getKnownMinValue() != 1 || ST.getELEN() == 64));
}
```
I haven't delved too deep but I think it has to do with the temporal behavior of `std::initializer_list<LLT>`. My reasoning is that the temporary array created for nitializer_list<LLT> exists only during the initialization of `AllVecTypes`. Once the initializer list has been used to initialize it, the temporary array goes out of scope, and the elements of `AllVecTypes` are left pointing to somewhere unknown.

What's your insight into this behavior and what's your opinion on implementation @michaelmaitland @topperc ? I mean using them in-place works, but it's clumsy.

https://github.com/llvm/llvm-project/pull/71400