[SPIR-V] Cast ptr kernel args to i8* when used as Store's value operand (#78603)

Handle a special case when StoreInst's value operand is a kernel
argument of a pointer type. Since these arguments could have either a
basic element type (e.g. float*) or OpenCL builtin type (sampler_t),
bitcast the StoreInst's value operand to default pointer element type
(i8).

This pull request addresses the issue
https://github.com/llvm/llvm-project/issues/72864