nir/spirv: Don't assert that the current block is empty
nir/spirv: Don't assert that the current block is empty

It's possible that someone will give us SPIR-V code in which someone
needlessly branches to new blocks.  We should handle that ok now.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler