nir: Fix OpAtomicCounterIDecrement for uniform atomic counters

nir: Fix OpAtomicCounterIDecrement for uniform atomic counters

From the SPIR-V 1.0 specification, section 3.32.18, "Atomic
Instructions":

   "OpAtomicIDecrement:
    <skip>
    The instruction's result is the Original Value."

However, we were implementing it, for uniform atomic counters, as a
pre-decrement operation, as was the one available from GLSL.

Renamed the former nir intrinsic 'atomic_counter_dec*' to
'atomic_counter_pre_dec*' for clarification purposes, as it implements
a pre-decrement operation as specified for GLSL. From GLSL 4.50 spec,
section 8.10, "Atomic Counter Functions":

   "uint atomicCounterDecrement (atomic_uint c)

    Atomically
    1. decrements the counter for c, and
    2. returns the value resulting from the decrement operation.

    These two steps are done atomically with respect to the atomic
    counter functions in this table."

Added a new nir intrinsic 'atomic_counter_post_dec*' which implements
a post-decrement operation as required by SPIR-V.

v2: (Timothy Arceri)
   * Add extra spec quotes on commit message
   * Use "post" instead "pos" to avoid confusion with "position"

Signed-off-by: Antia Puentes <apuentes@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>