[SPIR-V] Fix OpVariable instructions place in a function (#87554)

This PR:
* fixes OpVariable instructions place in a function (see
https://github.com/llvm/llvm-project/issues/66261),
* improves type inference,
* helps avoiding unneeded bitcasts when validating function call's

This allows to improve existing and add new test cases with more strict
checks. OpVariable fix refers to "All OpVariable instructions in a
function must be the first instructions in the first block" requirement
from SPIR-V spec.