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

[Jay Foad via llvm-commits]

jayfoad 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.

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