Move folding routines into a class

The folding routines are currently global functions.  They also rely on
data in an std::map that holds the folding rules for each opcode.  This
causes that map to not have a clear owner, and therefore never gets
deleted.

There has been a request to delete this map.  To implement this, we will
create a InstructionFolder class that owns the maps.  The IRContext will
own the InstructionFolder instance.  Then the global functions will
become public memeber functions of the InstructionFolder.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1659.