freedreno/ir3: Use third register for offset for LDL and LDLV

freedreno/ir3: Use third register for offset for LDL and LDLV

Before, offset held the offset, which can be either immediate or a
register.  Use a third register to hold the offset so that we can use
a register.

Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>