[SPIR-V]: add SPIR-V extension: SPV_INTEL_variable_length_array (#83002)

This PR adds SPIR-V extension SPV_INTEL_variable_length_array that
allows to allocate local arrays whose number of elements is unknown at
compile time:
* add a new SPIR-V internal intrinsic:int_spv_alloca_array
* legalize G_STACKSAVE and G_STACKRESTORE
* implement allocation of arrays (previously getArraySize() of
AllocaInst was not used)
* add tests