[mlir][spirv] Relax verification to allow flexible placement

Thus far certain SPIR-V ops have been required to be in spv.module.
While this provides strong verification to catch unexpected errors,
it's quite rigid and makes progressive lowering difficult. Sometimes
we would like to partially lower ops from other dialects, which may
involve creating ops like global variables that should be placed in
other module-like ops. So this commit relaxes the requirement of
such SPIR-V ops' scope to module-like ops. Similarly for function-
like ops.

Differential Revision: https://reviews.llvm.org/D73415