spirv-val: Get things to cases for OpSpecConstantOp (KhronosGroup#6579)

For the "final prep" for
KhronosGroup#6564

Made a commit per file (incase people have fun with merge conflicts) to
make main pass function call into smaller functions, this will allow for
the `OpSpecConstantOp` usage to work as well

This was a pure copy-and-paste PR

Author: spencer-lunarg