freedreno/ir3: add meta instruction for pre-fs texture fetch

freedreno/ir3: add meta instruction for pre-fs texture fetch

Add a placeholder instruction to track texture fetches made prior to FS
shader dispatch.  These, like meta:input instructions are scheduled
before any real instructions, so that RA realizes their result values
are live before the first real instruction.  And to give legalize a way
to track usage of fetched sample requiring (sy) sync flags.

There is some related special handling for varying texcoord inputs used
for pre-fs-fetch, so that they are not DCE'd and remain in linkage
between FS and previous stage.  Note that we could almost avoid this
special handling by giving meta:tex_prefetch real src arguments, except
that in the FS stage, inputs are actual bary.f/ldlv instructions.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>