[AMDGPU][MC][GFX8+] Add a symbolic name for 1/2pi

[dpreobra]

Bugzilla Link	39296
Version	trunk
OS	All
CC	@atamazov,@arsenm

Extended Description

Currently 1/2pi may only be specified in a numeric form. This is inconvenient and error-prone. Examples:

v_add_f16 v0, 0.15915494, v1
v_add_f64 v[0:1], v[2:3], 0.159154943091895317852646485335

Assembler should support special syntax or a symbolic name for this constant. For example:

v_add_f16 v0, INV2PI, v1
v_add_f64 v[0:1], v[2:3], INV2PI

Any suggestions?

[dpreobra]

assigned to @dpreobra

[jwanggit86]

PR 160617 was created. However, reviewers were in general against introducing the symbolic name. One good point is that asm programmers can use macros to achieve what' proposed here without having the compiler support it.