Upgrade modf and frexp (#2266)

Fixes #2138

* Modf and frexp are upgraded to use the struct version of the
instruction and generate an explicit store whose flags can be upgraded
separately
* Fixed major bug where availability and visibility were reversed for
non-copy memory instructions
* Fixed bug where availability and visibility scope operands were reversed for copy memory
* Upgraded all opt tests to use SPV_ENV_UNIVERSAL_1_3
* Upgrade tests moved into unified tests and removed standalone test