nir/builder: Add a helper for getting the most recently added instruction
nir/builder: Add a helper for getting the most recently added instruction

Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler