[SPIRV] Add OpAccessChain instruction support (#66253)

This commit adds 2 new instructions in the selector:
 - OpAccessChain
 - OpInBoundsAccessChain.

The choice between the two relies on the `inbounds` marker.

Those instruction are not used for OpenCL, to maintain the same
behavior as previously. They are only added when building for logical
SPIR-V, as it doesn't support the pointer equivalent.

Because logical SPIR-V doesn't support pointer cast either, the
assign_ptr_type intrinsic need to be generated so OpAccessChain gets
lowered with the correct pointer type, instead of i8*.

Fixes #66107

---------

Signed-off-by: Nathan Gauër <brioche@google.com>