aco: consider how definitions fixed to operands can change register demand

aco: consider how definitions fixed to operands can change register demand

Usually the register demand before an instruction would be considered part
of the previous instruction, since it's not greater than the register
demand for that previous instruction. Except, it can be greater in the
case of an definition fixed to a non-killed operand: the RA needs to
reserve space between the two instructions for the definition (containing
a copy of the operand).

fossil-db (navi21):
Totals from 5 (0.00% of 135636) affected shaders:
PreVGPRs: 35 -> 40 (+14.29%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8807
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22446>