Allow entry points with missing numthreads on CPU targets (#8678)

Several tests have compute entry points without a `[numthreads(x,y,z)]`
decoration. Currently, none of these tests run on the CPU target, as
they crash the compiler. I took a look at the SPIR-V emitter, which
falls back to a workgroup size of (1,1,1):


https://github.com/shader-slang/slang/blob/1e0908bd7107dfbdac912b693c3ab9bd6e1dc8b3/source/slang/slang-ir-spirv-legalize.cpp#L1635-L1643

To match this behaviour, this PR implements a fallback solution that
makes `emitCalcGroupExtents()` emit (1,1,1).

This PR is both a question and a suggestion; I'm not sure the approach
here is at all reasonable. Personally, I'd just like to explicitly add
`[numthreads(1,1,1)]` to all such tests, but I don't know if it's
actually legal and supported to not have a `numthreads`. So the
implementation here is a bit conservative.

I ran across these when I went through tests for the upcoming LLVM
target. These were the final blockers to get all autodiff and
language-features tests passing (not counting the ones using things like
wave intrinsics and barriers etc.)

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