DXC Validator doesn't error on out of bounds access at index -1

[spall]

The validator doesn't catch '-1' used as a vector index:
https://hlsl.godbolt.org/z/MoeTbjnz4

Expected behavior:
error: validation errors

:5:10: error: Access to out-of-bounds memory is disallowed. note: at '%6 = getelementptr [2 x i32], [2 x i32]* %1, i32 0, i32 -1' in block '#0' of function '�?foo@@yahhh@Z'. Validation failed.