[AMDGPU] Allow frontends to disable null export for pixel shaders

Disable null export (for kills) when a frontend defines a pixel
shader as not exporting using amdgpu-color-export and
amdgpu-depth-export function attrbutes.
This allows the generation of export free pixel shaders.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D105683