freedreno/a3xx/compiler: cat4 cannot use const reg as src

freedreno/a3xx/compiler: cat4 cannot use const reg as src

Category 4 instructions (rsq, rcp, sqrt, etc) seem to be unable to take
a const register as src.  In these cases we need to move the src to a
temporary gpr first.

This is the second case of such a restriction, where the instruction
encoding appears to support a const src, but in fact the hw appears to
ignore that bit.  So split things out into a helper that can be re-used
for any instructions which have this limitation.

Signed-off-by: Rob Clark <robclark@freedesktop.org>