freedreno/ir3: Fix up the half reg source even when src instr==NULL

freedreno/ir3: Fix up the half reg source even when src instr==NULL

Previously the loop for assigning registers was bailing out early if
the register had a null source. I think the intention is that in this
case it isn’t necessary to assign a register. However it was also
missing out the part to fix up the types. This can happen if the
instruction is copy propagated to be a move from a constant half-float
input register. In that case it still needs to fix up the types.

Fixes assert in
dEQP-GLES3.functional.shaders.invariance.highp.subexpression_precision_mediump

when lowering the precision of the variables.

Signed-off-by: Rob Clark <robdclark@chromium.org>