[SPIRV] Don't lower builtin variadic functions (#188517)
In https://github.com/llvm/llvm-project/pull/178980 I tried to remove
the special handling for `printf`, but the fix was wrong, the pass
expects that `printf` abides by the variadic ABI (single buffer passed
in, args extracted by caller), which isn't how it works.
However the root issue is with any builtin, they are just replaced
directly with instructions, and the only place that could generate code
to honor the variadic ABI would bei in the SPIR-V backend, and it would
be pretty complicated for pretty much no benefit.
It's much simpler to teach the pass to skip certain functions, as we did
before, but this time skip all SPIR-V builtin.
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Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
