[SPIR-V] Fix OpDecorate emission after vreg def. (#114426)
In SPIR-V, OpDecorate instructions are allowed to forward-declare a
virtual register. But while we are at the MIR level, we must comply with
stricter rules, meaning OpDecorate should be emited after, not before
the reg definition.
(In some cases, we defined those just before, switching to just after).
Related to #110652
---------
Signed-off-by: Nathan Gauër <brioche@google.com>
