vc4: Uniform lowering should insert instructions in the idom.

[anholt]

VC4 can't have two uniform reads in the same instruction. vc4_qir_lower_uniforms.c looks at the instructions using two uniforms and decides which uniform values should be moved to a separate MOV instruction. It tries to insert a minimal number of MOVs to get the job done, but unfortunately it inserts those MOVs at the top instead of in the idom of the instructions using them. This causes register allocation failures sometimes, particularly in conformance tests.

We should either insert into the idom, or insert separate MOVs in each basic block to reduce register pressure.