Fix crash in Spirv -lower-host-to-llvm pass

When providing with a spirv module as input where no conversion happens
the code didn't defend against broken invariant.

We'll fail the pass here, but it's not clear if it is the right thing
or if the module should just be ignored.

Fixes #59971

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D141856