[AMDGPU] Inline non-kernel functions using extern lds

In https://reviews.llvm.org/D100481, forceful inline of all non-kernel
functions using lds was disabled since AMDGPULowerModuleLDS pass now handles
static lds. However that pass does not handle extern lds so non-kernel
functions using extern lds must sill be inline.

Reviewed By: hsmhsm, arsenm

Differential Revision: https://reviews.llvm.org/D109773