nir: Rename is_arb_asm to use_legacy_math_rules and document its meaning.

On iris and crocus, this flag is used to set "alt mode" math on the shader
as a whole.  Some other drivers have a similar mode for DX9/ARB-program
behavior, so document what it does so we can start using it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16176>

src/compiler/glsl/standalone.cpp

@@ -102,7 +102,7 @@ init_gl_program(struct gl_program *prog, bool is_arb_asm, gl_shader_stage stage)
 {
    prog->RefCount = 1;
    prog->Format = GL_PROGRAM_FORMAT_ASCII_ARB;
-   prog->info.is_arb_asm = is_arb_asm;
+   prog->info.use_legacy_math_rules = is_arb_asm;
    prog->info.stage = stage;
 }