[llvm] [AMDGPU] Add note about inactive VGPR lanes being preserved (PR #185672)

[Diana Picus via llvm-commits]

rovka wrote:

> Also I notice that this is already mentioned in the section "Code Conventions" -> "AMDHSA" -> "Call Convention" -> "Non-Kernel Functions". I am not sure how we came to have calling convention documentation in two different places in this document.

I think it would be nice to move things out of AMDHSA, because the calling conventions shouldn't behave differently depending on whether the triple is amdhsa or amdpal or something else. They're kind of their own thing, and then the AMDHSA, AMDPAL and whatever other sections can just mention which calling conventions are relevant to them and link to where they're described. 

https://github.com/llvm/llvm-project/pull/185672