[AMDGPU] Run LowerLDS at the end of the fullLTO pipeline (#75333)

This change allows us to use `--lto-partitions` in some cases (not at
all guaranteed it works perfectly), as LDS is lowered before the module
is split for parallel codegen.

We must run LowerLDS before splitting modules as it needs to see all
callers of functions with LDS to properly lower them.