spirv: Rework error checking for decorations

spirv: Rework error checking for decorations

This reworks the error checking on our generic handling of decorations.
The objective is to validate all of the SPIR-V assumptions we make
up-front and convert redundant checks to compiled-out asserts.  The most
important part of this is to ensure that member decorations only occur
on OpTypeStruct and that the member is never out-of-bounds.  This way
later code can assume that the member is sane and not have to worry
about OOB array access due to a misplaced OpMemberDecorate.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>