spirv: Use nir_imm_floatN_t for constants for GLSL450 builtins

There is an existing macro that is used to choose between either a
float or a double immediate constant based on the bit size of the
first operand to the builtin. This is now changed to use the new
nir_imm_floatN_t helper function to reduce the number of places that
make this decision.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/spirv/vtn_glsl450.c

@@ -514,7 +514,7 @@ vtn_nir_alu_op_for_spirv_glsl_opcode(struct vtn_builder *b,
    }
 }
 
-#define NIR_IMM_FP(n, v) (src[0]->bit_size == 64 ? nir_imm_double(n, v) : nir_imm_float(n, v))
+#define NIR_IMM_FP(n, v) (nir_imm_floatN_t(n, v, src[0]->bit_size))
 
 static void
 handle_glsl450_alu(struct vtn_builder *b, enum GLSLstd450 entrypoint,